Plotting problem in pandas 0.15.1 for DataFrame with datetime-index
Hi have the following snippet:

```
from datetime import datetime
import matplotlib.pyplot as plt
import numpy as np
import pandas as pd

plot_df = pd.DataFrame(
    np.random.rand(21, 2),
    index=pd.bdate_range(datetime(2000, 1, 1), datetime(2000, 1, 31)),
    columns=['a', 'b'])

fig = plt.figure()
ax = fig.add_subplot(111)
ax = plot_df.plot(ax=ax)
plt.axhline(y=0)
```

This gives an exception in Pandas 0.15.2, **not** in Pandas 0.14.1.
Trace:

```
Traceback (most recent call last):
  File "test_p15.py", line 14, in <module>
    plt.axhline(y=0)
  File "/Users/dcoevord/anaconda/envs/p15/lib/python2.7/site-packages/matplotlib/pyplot.py", line 2501, in axhline
    ret = ax.axhline(y=y, xmin=xmin, xmax=xmax, **kwargs)
  File "/Users/dcoevord/anaconda/envs/p15/lib/python2.7/site-packages/matplotlib/axes/_axes.py", line 729, in axhline
    self.add_line(l)
  File "/Users/dcoevord/anaconda/envs/p15/lib/python2.7/site-packages/matplotlib/axes/_base.py", line 1486, in add_line
    self._update_line_limits(line)
  File "/Users/dcoevord/anaconda/envs/p15/lib/python2.7/site-packages/matplotlib/axes/_base.py", line 1497, in _update_line_limits
    path = line.get_path()
  File "/Users/dcoevord/anaconda/envs/p15/lib/python2.7/site-packages/matplotlib/lines.py", line 871, in get_path
    self.recache()
  File "/Users/dcoevord/anaconda/envs/p15/lib/python2.7/site-packages/matplotlib/lines.py", line 568, in recache
    xconv = self.convert_xunits(self._xorig)
  File "/Users/dcoevord/anaconda/envs/p15/lib/python2.7/site-packages/matplotlib/artist.py", line 163, in convert_xunits
    return ax.xaxis.convert_units(x)
  File "/Users/dcoevord/anaconda/envs/p15/lib/python2.7/site-packages/matplotlib/axis.py", line 1448, in convert_units
    ret = self.converter.convert(x, self.units, self)
  File "/Users/dcoevord/anaconda/envs/p15/lib/python2.7/site-packages/pandas/tseries/converter.py", line 121, in convert
    return PeriodIndex(values, freq=axis.freq).values
  File "/Users/dcoevord/anaconda/envs/p15/lib/python2.7/site-packages/pandas/tseries/period.py", line 641, in __new__
    ordinal, freq = cls._from_arraylike(data, freq, tz)
  File "/Users/dcoevord/anaconda/envs/p15/lib/python2.7/site-packages/pandas/tseries/period.py", line 691, in _from_arraylike
    data = _get_ordinals(data, freq)
  File "/Users/dcoevord/anaconda/envs/p15/lib/python2.7/site-packages/pandas/tseries/period.py", line 507, in _get_ordinals
    return lib.map_infer(data, f)
  File "pandas/src/inference.pyx", line 1020, in pandas.lib.map_infer (pandas/lib.c:56502)
  File "/Users/dcoevord/anaconda/envs/p15/lib/python2.7/site-packages/pandas/tseries/period.py", line 503, in <lambda>
    f = lambda x: Period(x, freq=freq).ordinal
  File "/Users/dcoevord/anaconda/envs/p15/lib/python2.7/site-packages/pandas/tseries/period.py", line 126, in __init__
    dt, _, reso = parse_time_string(value, freq)
  File "/Users/dcoevord/anaconda/envs/p15/lib/python2.7/site-packages/pandas/tseries/tools.py", line 460, in parse_time_string
    raise DateParseError(e)
pandas.tseries.tools.DateParseError: day is out of range for month
```

Version info:

```
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.8.final.0
python-bits: 64
OS: Darwin
OS-release: 14.0.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: None

pandas: 0.15.1
nose: None
Cython: None
numpy: 1.9.1
scipy: 0.14.0
statsmodels: None
IPython: 2.3.1
sphinx: None
patsy: None
dateutil: 1.5
pytz: 2014.9
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.4.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
rpy2: None
sqlalchemy: None
pymysql: None
psycopg2: None
```
