Add getMilestoneList and showMilestoneList

add complementary vim functions
Related issue #83 #31
Set/update milestone if necessary
Fully support milestones
Clean up the API a little bit
Enforce a default state for Milestone

This allows us to remove the milestone by blanking it out
Add more error messages

Update #2 Need to add error messages, fallbacks
Merge branch 'gh3.5'

Conflicts:
	autoload/ghissues.vim
Last batch of error handlers

Close #2 Need to add error messages, fallbacks
Fix bad merge