Handle between_time on empty series
Right now using between_time on an empty series will raise an TypeError: Index must be DatetimeIndex. I would think that it would make more sense to return an empty list (or at least return a more helpful Error).

``` python
import datetime as dt
import pandas as pd
s = pd.Series([])
s.between_time(dt.time(0, 0), dt.time(23, 0))
```

I would assume that expanding the Error Handler of the method should do the trick:

``` python
[...]
except AttributeError:
    if self.empty:
        return self
    else:
         raise TypeError('Index must be DatetimeIndex')
```

The Error persists on this configuration:
INSTALLED VERSIONS
commit: None
python: 2.7.6.final.0
python-bits: 32
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 58 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None

pandas: 0.16.2
nose: 1.3.3
Cython: 0.20.1
numpy: 1.8.1
scipy: 0.14.0
statsmodels: 0.5.0
IPython: 2.1.0
sphinx: 1.2.2
patsy: 0.2.1
dateutil: 2.2
pytz: 2014.3
bottleneck: 0.8.0
tables: 3.1.1
numexpr: 2.4
matplotlib: 1.4.0
openpyxl: 2.0.3
xlrd: 0.9.3
xlwt: 0.7.5
xlsxwriter: None
lxml: 3.3.5
bs4: 4.3.2
html5lib: 0.999
httplib2: None
apiclient: None
sqlalchemy: 0.9.4
pymysql: None
psycopg2: None
