[Emacs mode] Make tern-command default to ../bin/tern if it exists

(Tip from Mihai Bazon)
Add parens around a regexp to help out js-mode's braindead tokenizer
Define a list-of-changes format for transformations computed server-side

Hopefully use this to write auto-refactor logic only once, rather
than once for every client.
[Emacs mode] Add tern-rename-variable