Add GitHub Enterprise, assignee and issue state

[ojacques]

Thanks for this nice little tool!
Here is my contribution:

• Add support for GitHub enterprise instance
• Add support for assignee
• Add support for "closed" state issues, using a 2 step processes where we first create the issue, then close it (useful for historical purposes when moving out from another issue tracker)
• Be a little bit more gentle on GitHub API (even slower rate throttling)
• Fixed a bug where token parameter was not taken into account on command line

[gavinr]

@ojacques Merged these changes, and about to release under a new version. Thanks a lot!