[Chart] First batch of patches from Jan Bracker's GSOC project

[Tim Docker]

I'm pleased to let you know that the a (very!) large set of patches have 
been merged to the chart library's master branch.

This is a large refactor and reorganisation that:

     - refines the minimum set of drawing operations required to render 
charts
     - abstracts the drawing backend with a clever use of the 
"operational monad" package
     - moves the existing cairo dependency into a separate chart-cairo 
package
     - adds an (as yet incomplete) backend package for drawing charts 
with the diagrams library
     - cleans up a lot of code
     - adds a lot of in-line documentation

Whilst the backend abstraction is the main goal, I think the overall 
quality of the codebase has improved significantly with these changes.

Jan has put a lot of work into this so far - Thanks Jan!

I expect that this will be the bulk of the GSOC related changes to the 
chart library itself. However, there is a proposal to switch from 
data-accessor to lens for field access. Given such a change will result 
in a significant API breakage, I'll hold off releasing current master as 
a new hackage release until a decision on lens is made.

Tim