Add GitHub Enterprise support and API token authentication.

GitHub Enterprise support is supported with the addition of GITHUB_API_BASE in
settings.py (also supported per repo as github_api_base). This option is set to
the API base URL. For github.com, this is "https://api.github.com" and that is
the default setting. For a GitHub Enterprise installation, this can be changed
to point to an internal server. E.g. "http://github.example.com/api/v3".

Some GitHub Enterprise installations operate with a self-signed certificate, so
a GITHUB_VERIFY option is also added. This value should always be set to True
unless GITHUB_API_BASE points to an internal GitHub Enterprise using a self-
signed certificate. This option is also allowed as a repo configuration option.

Also, GITHUB_TOKEN (github_token) was also added to allow authentication via
an API token instead of using a username and password in settings.py. API
tokens can be revoked easily and thus are more secure. Also, in GitHub
Enterprise, additional users cost real money, so being able to use a token
instead of creating a 'bot' user is beneficial.
Fix _parse_jenkins_json to handle application/json content-type.

With Jenkins Notification plugin 1.5, the content-type appears to be set
correctly to application/json. This change should work with previous versions
that set it to application/x-www-form-urlencoded.
Initiate build when PR is reopened.

When a PR has been reopened, kick off another build for the commit(s). This
also helps address issue #9 as you can close/reopen a PR to force a rebuild.
Merge branch 'build_on_reopen'
Merge branch 'github_enterprise'