Remove non-standard/unimplemented short API
Add driver selection (close #18)

Currently, this won't complain if you try to load a driver that won't
work in the browser loaded, so we should probably cover that off before this
gets merged in.
Don't load incompatible driver
Use lowercase filenames for sanity
Fix weird extra require
Add example to README and clear out TODO about driver selection
Use async loading when needed
Merge pull request #33 from mozilla/driver-selection

Driver selection