BREAKING: refactor renderer connections

[guruofgentoo]

Currently, with the way Excel exports work, format-specific information has a lot of fingers reaching into the main grid code.

• there are methods in the framework plugins and the HTML renderer directly named for an xls export
• columns have xls-specific init parameters and methods
• Grid.set_renderers sets direct attributes for the renderers (e.g. Grid.html, Grid.xls)

This needs to get refactored. When #21 happens, the first point above should be addressed in a non-breaking manner. The other items will break compatibility and should be reserved for a more major revision:

• Grid should namespace renderer attributes set in set_renderers
• the xls-specific methods in columns will need to be thought through a bit more. Renderer-specific operations should be handled within the renderer, which may mean passing an inner class of the renderer to the column to keep track of those things.