add poltergeist
add dependencies phantomjs

@KitaitiMakoto @shikakun

when this branch is merged, you need to install phantomjs
See: https://github.com/jonleighton/poltergeist#installing-phantomjs
add poltergeist settings
this is minor fix, and irrelavant of this pr though :P
deletable snippet only if snippet owner
there are no need to use `let!`
Merge pull request #27 from iiirc/deletable-snippet

Deletable snippet
Make app return 404 when snippet not exist
Merge pull request #29 from iiirc/access-to-deleted-snippets

Make app return 404 when snippet not exist
Merge branch 'master' into user-snippet-list

Conflicts:
	app/controllers/snippets_controller.rb