Rename CacheableWrapper to Reflectable.
Rename WrapperCache to Reflector.

I think the abstraction from the raw JSObject is still probably worthwhile for
now.
Rename get_wrappercache() to reflector(). v1

I also updated some variable names in the codegen.
Rename {get,set}_wrapper to {get,set}_jsobject. v1
Rename WrappableDocument stuff.
Rename some local variables and shorten some lines.

This was done manually, and might benefit from a more careful review.
Rename the |wrapper| field in Reflector.
Rename |wrapper| to |reflector_| in Reflectable implementations for DOM objects.
auto merge of #1028 : bholley/servo/reflector_rename, r=jdm

There are still a few instances of "wrapper"-ish names scattered throughout the code, but this is a good start.