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
add changelog
update Gemfile.lock
Link to Getting Started Wiki Page in README
Add "Running Pageflow" to README
change quota api

* distinguish exhausted and exceeded quotas
* model quotas as inidividual objects by restricted trait
add editor routing constraint

allow restricting access to the admin and editor routes defined by
pageflow through a routing contstraint.
change hooks api

do not proxy Hooks#on method on configuration.
export factory girl factories

projects using the engine should not need to define their own
factories for pageflow models.
add missing QuotaDouble
refs #9556 fixes verweisseiten thumbnails not showing on public page
introduce theming model
add some translations