Reorganize to allow for alternate datasources

This change is intended to provide a way to implement alternate backends for tasseo.

The javascript has been split into 3 classes:

1. `TasseoDashboard` which is responsible for a collection of graphs
2. `TasseoGraphiteDatasource` which is responsible for fetching data from graphite
3. `TasseoUi` which is responsible for switching periods as well as day/night mode

Much of the code that was in `j/tasseo.js` that used global variables to configure aspects of operation have been moved to `index.haml` to allow for `j/tasseo.js` to be loaded in places that need more control over operation.