Add recursive dropdown for sentence elements
Properly delegate dropdown menu events
Create common button style
Fix various minor design issues
Replace common colors and margins with variables
Move refresh event handler to button element
Finalize JS export wrapper

The exported JS code is importable with CommonJS, AMD, or via the Baba
global browser object. Exported sentences are stored as Baba object
properties, and are generated by camelcasing the node name.

The export functionality can be tested by running e.g.:

    (1, eval)(exportGrammar.exportToBrowser(vm))
    console.log(Baba.testSentence())

This correctly returns a random sentence, but stuff like transforms and
whitespace aren't implemented yet, so the sentence looks something like
"weirdcatisbeatfoot" instead of "a weird cat is beating a foot".

The overhead added to the grammar (module export, sentence parsing) is
minimal, it's currently about 0.5kB un-gzipped, and will probably remain
at <1kB when complete.