Add support for exporting from github.

This commit adds undocumented support for exporting from github-based
issue trackers via the Github v3 API.  The syntax for this looks like:

bugging-out --type github --server https://api.github.com --owner \
    gerlowskija --project bugging-out

Data is currently exported as JSON by default, and hasn't been checked
for data-parity.  By 'data-parity', I mean that I haven't yet checked
whether the Github API returns the same fields about an issue that
the JIRA API does.

Issue #7