Series.rank() doesn't handle small floats correctly
Floats below 1e-10 seem to all be receiving the same rank, incorrectly:

```
In [1]: import pandas

In [3]: import numpy

In [4]: series = pandas.Series([1e-100, 1e-25, 1e-20, 1e-15, 1e-10, 
                                1e-5, 1e-4, 1e-3, 1e-2, 1e-1])

In [5]: series
Out[5]: 
0    1.000000e-100
1     1.000000e-25
2     1.000000e-20
3     1.000000e-15
4     1.000000e-10
5     1.000000e-05
6     1.000000e-04
7     1.000000e-03
8     1.000000e-02
9     1.000000e-01
dtype: float64

In [6]: series.rank()
Out[6]: 
0     2.5
1     2.5
2     2.5
3     2.5
4     5.0
5     6.0
6     7.0
7     8.0
8     9.0
9    10.0
dtype: float64

In [7]: from scipy import stats

In [8]: stats.rankdata(series)
Out[8]: array([  1.,   2.,   3.,   4.,   5.,   6.,   7.,   8.,   9.,  10.])

In [13]: pandas.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.3.final.0
python-bits: 64
OS: Darwin
OS-release: 10.8.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.13.1
Cython: 0.19.1
numpy: 1.8.0
scipy: 0.12.0.dev-1d5c886
statsmodels: 0.5.0
IPython: 1.2.1
sphinx: 1.2.2
patsy: 0.2.0
scikits.timeseries: None
dateutil: 2.2
pytz: 2013b
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.2.0
openpyxl: 1.5.7
xlrd: 0.7.1
xlwt: None
xlsxwriter: None
sqlalchemy: 0.6.6
lxml: None
bs4: None
html5lib: None
bq: None
apiclient: None
```
