Subtract Timestamp from DatetimeIndex
A TypeError is raised when attempting to subtract a Timestamp from a DatetimeIndex when the result of the subtraction is zero.

```
dr = pd.date_range('20140101', freq='S', periods=11)
zero = dr[5]
diff = dr - zero
```

raises a TypeError, however,

```
diff = [i - zero for i in dr]
```

produces the desired list of timedelta objects.

```
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.6.final.0
python-bits: 64
OS: Darwin
OS-release: 12.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.13.1
Cython: 0.20.1
numpy: 1.8.0
scipy: 0.13.3
statsmodels: None
IPython: 2.0.0
sphinx: 1.2.2
patsy: None
scikits.timeseries: None
dateutil: 2.2
pytz: 2013.9
bottleneck: 0.6.0
tables: 3.1.1
numexpr: 2.1
matplotlib: 1.3.1
openpyxl: None
xlrd: 0.9.2
xlwt: None
xlsxwriter: None
sqlalchemy: None
lxml: None
bs4: None
html5lib: None
bq: None
apiclient: None
```
