v2

* thor + octokit
* --all option (auto pagination)
* new `ghn open` command (--open option is osbolete)
spec
pre1
Add aliases
doc
Merge branch 'v2'
Don't raise exception for unknown subject type notification
Remove `nil` from collected URLs
Merge branch 'fix-raise-for-unknown-type'
`__dir__` is not compatible for Ruby 1.9.x
Merge branch 'ruby19'
Fix build

- Drop rbx
- Add 2.1.1
Merge branch 'fix-travis-build'
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
Merge branch 'release_type'
Bump version
Merge pull request #27 from syohex/support-other-than-macosx

Support platforms other than MacOSX