add hooks to config

subscribing to and invoking hooks
invoke hooks when files are submitted and encoded
user quotas for accounts

* add configurable quota adapter
* prevent user invitation if users quota is exceeded
published entries quotas for accounts
check published entries quota in editor

* check before displaying PublishEntryView
* render custom html partial if quota is exceeded
install spring rspec commands
asset precompilation and some path fixes (#6)
make user invitation mailer sender address configurable