Enable histogram panel to plot multiple fields

Closes https://github.com/elasticsearch/kibana/issues/199

This is a first prototype to enable histogram panel to plot multiple fields. It also enables the pairing of a "plot" (a line in the graph) to a set of queries.
Thus it introduces a new concept to the panel: a plot, which is represented by one (or more) lines on the graph. It is associated with: a mode (same as before), a value field (when the mode is different than 'count'), and a set of query ('all','pinned','unpinned','selected'). The default color of a line is taken from the query it is paired to, but it can be changed for each plot. A plot can also have an alias, cause a pair (queryA, value_fieldA) may represents something different than the pair (queryA, value_fieldB).

Worth noting that the time_field is shared across all plots.

How the panel looks like: (left panel=>first query, right panel=>second query)
![b001](https://f.cloud.github.com/assets/5235401/968642/f6c884ac-05a7-11e3-8153-3fc0e822a429.png)

How the editor looks like: (editor for the second panel)
![b002](https://f.cloud.github.com/assets/5235401/968645/fc30105e-05a7-11e3-9864-4f7701ea7713.png)