v2

* thor + octokit
* --all option (auto pagination)
* new `ghn open` command (--open option is osbolete)
spec
pre1
Add aliases
doc
Don't raise exception for unknown subject type notification
Remove `nil` from collected URLs
`__dir__` is not compatible for Ruby 1.9.x
Fix build

- Drop rbx
- Add 2.1.1
Bump version
Add Release type notification JSON payload and test case
Support platforms other than MacOSX

'open' command is a MacOSX specific command.
Implement subclasses
Bump version
Bump version
Make "follow issuecomment" feature optional
Update README
Bump version
Fix test
Add CONTRIBUTORS.md (Thank you very much!)