auto merge of #542 : eric93/servo/floats, r=pcwalton

I added the minimal amount of code needed to place left-floats on the screen (right floats should also be possible soon). Text does not wrap around floats yet.

One thing I'm curious about is whether some existing abstractions (like Cell) can be used instead of this weird overwriting thing done in float_context.rs.
Add right floats and fix bug where max_y was not being set