Add encoder to handle masked numbers.
Test json encoding for NaN, pd.NaT, pd.Index
If encoders aren't relevant, raise NotEncodable instead of returning None

Returning None for encoders failed in the case where the encodable value
was actually None, like pd.NaT -> None
:lipstick:
rm masked encoding encoder, as it's covered in the pandas, numpy, and datetime encoders
Check for pandas objects that subclass datetimes in the datetime encoder
Test that json encoding doesn't mutate
:lipstick:
import just once, and at the top
datetime any :lipstick;
document `to_plotly_json` encoder
Version bump (Improved pandas and JSON encoding)
Merge branch 'master' into jsonify-masked-numbers

Conflicts:
	plotly/version.py
Merge pull request #159 from plotly/jsonify-masked-numbers

Add encoder to handle masked numbers.