Background and grid support in mpl.

The changes are simple (it takes me some time to deal with the matplotlib API - not working right for grid lines, a story for another day :laughing:)... but the results are much better with these changes.
As examples I will add two pics I pasted on FD .

![grid](https://cloud.githubusercontent.com/assets/1640669/2620224/f7fedef8-bc3a-11e3-894b-a61332fb58c0.png)

![violin](https://cloud.githubusercontent.com/assets/1640669/2620284/cd346d7c-bc3b-11e3-8f5f-04aec5aa2c9f.png)