Factor out validation and conversion code.

Use the same validation code in the bulk_write method.
Convert input data to pandas.TimeSeries for writing.
Simplify writer method.
Use pandas to enforce sequential timeseries.

Move assert into validation method. Use pd.asfreq.
Underlying code to specify freq other than daily.

Working towards supporting frequencies of data other than daily.
Move sphinx-apidoc execution into sphinx conf.py.

This means only executing sphinx-build should also result in
auto-generated api documentation.

This should work better with Read The Docs if we use it.