Add helper functions for floats
Added floats to the flow tree
Allow floats to have specified heights
Fix up problems with float placement
Stop crashing when building the flow tree
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.
Create text runs during box building