Support github public and github enterprise in the same time

issue mentioned here: https://github.com/ehamiter/ST2-GitHubinator/issues/29

Two things are done here:
1. load "DEFAULT_GITHUB_HOST" from `.git/config` if find "https://" or "http://" url description
2. support "http://" style github url

PS:
My Github enterprise environment only use the https:// or http:// github style, so this PR is enough for me, but it's not robust for all Github enterprise scenarios.