use oauth and access token for authorization

Username and password are only used to request token.  Token
is then stored in the `.env` file.  There are two benefits:

 1. Keeps your github password off disk.
 2. Token has no [scopes](http://developer.github.com/v3/oauth/#scopes)
    set so it has fewer privileges than basic authentication.

If you don't want to give local-pulse your password, you can
create a token at https://github.com/settings/tokens/new and
manually create a `.env` file like:

    GITHUB_OAUTH_TOKEN=40_character_token_created_via_url_above
    GITHUB_LOCATION=indianapolis, ohio, new york
Merge pull request #4 from bfritz/oauth-login

use oauth and access token for authorization