pandas.cut crashes on all zero input
xref #7751 

``` python
import pandas as pd
pd.cut([0,0,0,0], 10)
```

result:

```
ValueError: Bin edges must be unique: array([ 0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.])
```

On the other hand, pandas.cut works on non-unique inputs other than zero.  The culprit are the following [lines of code in tile.py](https://github.com/pydata/pandas/blob/v0.14.1/pandas/tools/tile.py#L89-L90):

``` python
        if mn == mx:  # adjust end points before binning
            mn -= .001 * mn
            mx += .001 * mx
            bins = np.linspace(mn, mx, bins + 1, endpoint=True)
```

When `mn` and `mx` are zero, the adjustment isn't made.

---

```
>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.6.final.0
python-bits: 64
OS: Linux
OS-release: 2.6.32-300.3.1.el6uek.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.14.1
nose: 1.3.1
Cython: None
numpy: 1.9.0
scipy: 0.14.0
statsmodels: None
IPython: 2.0.0
sphinx: None
patsy: None
scikits.timeseries: None
dateutil: 2.2
pytz: 2014.4
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.3.1
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
rpy2: 2.4.2
sqlalchemy: None
pymysql: None
psycopg2: None
```
