Mplot legend fix

The legend from `mprof plot` is displayed to the right of the main plot, this takes a lot of space. I have repositioned it so it is in the `best` placement with some alpha blending (in case it obscures part of the plot). This is more space efficient. Example:
![legend_best_place](https://f.cloud.github.com/assets/273210/2120655/fabf7c2a-91be-11e3-9332-86b78e7813e6.png)

I'm happy to take feedback on this, I'm using this approach for screenshots I've made for my book (because this tool is great for explaining memory usage!). I'd be happy to e.g. make this a command line option rather than just the default, I'm sharing this to start a conversation.