global-config

Store github authorization token globally
Fixes #33

## Changes:
* move oauth authorization token from local git config to `~/.config/gitx/github.yml` for reuse across projects
* mock out config file for tests by overriding `ENV['HOME']` to point to `spec/tmp`

## Note
This change will prompt all existing users to re-create their personal access token since the existing token stored in `.git/config` is no longer referenced.  There is no attempt to migrate the existing value.

<!---
@huboard:{"order":38.0,"milestone_order":38,"custom_state":""}
-->