Fix Dropbox typos - Githubissues

Fixes some typos in the documentation for #47:

There was a spurious reference to "Github" which should have been "Dropbox".
Callback URLs need to be absolute (My documentation had been written based on the support for relative callback URLs, which we ended up punting).
Changed docs to reflect the fact that a dropbox app can allow multiple callback URLs.

vapor-community / Imperial