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