Use Altair to encode data to the Vega Lite format

[synw]

Instead of a custom encoder use Altair to encode the data to the Vega Lite format. It is very convenient, for example to set encoding. I started to implemented this in the vegalite branch.

This means depending on altair and pandas: it sounds like the cleanest way to serialize the data. Anyway we would had to depend on pandas one day or the other, considering how good and widely used it is for the kind of tasks we are doing.

The declarative approach is really nice and we can still use other js libs to render the charts, isolating the boring imperative stuff in individual units for different rendering engines

[synw]

I updated the Vega Lite branch with two examples: one draws timeseries from a query (see this method) and the other draws a chart from a set of count queries, all using the auth.User model

[synw]

Done with merging the vegalite branch