Make Compose use Cairo package

This makes Compose use the Cairo.jl package allowing it to take advantage of perks such as automatic compilation/fetching of dependencies and integration with the rest of the Julia Graphics Infrastructure. Still has some few rough edges, mostly related to coordinate systems, but I don't know what the right fix here is to align that (whether to do it in Compose or Cairo). I think it's important to combine all of the Julia Graphics efforts and I think this is a first step towards that. 

The main reason for this change however is this:
![Screen Shot 2013-02-03 at 12 32 44 AM](https://f.cloud.github.com/assets/1291671/121583/2c4c6b88-6dc4-11e2-8959-58a6057f7f81.png)

Needing an external program to show plots sucks ;)!