Biggish refactor of dependencies

Got rid of all custom git dependencies and obliterated mysql and heap db.

The new fxa-auth-db-mem dev dep hosts a memory db over the httpdb
api.
LGTM, I ran `$ david` against your latest package.json and it only came back w/ two outdated dependencies:

> ### Outdated Dependencies
> - **jwcrypto** (package: 0.4.4, latest: 0.5.0)
> 
> ### Outdated Dev Dependencies
> - **ass** (package: 0.0.4, latest: 0.0.5)

Not sure if we want to bump **jwcrypto**.
I'm guessing we don't want to touch @lloyd's new and improved ass (can't remember if that was the one that added ass badges).