New features

Added the ability the define filters / virtual on the collection class level.
Fixed a bug in collection's 'clear' method
'get' - ability to specify a default value
Bug fix

Collection.fetch() should perform a smart merge, not a replaceall
Merge branch 'dev' of github.com:tkambler/bonegular into dev
Bug fix: collections being overwritten on fetch()
Don't delete virtual keys when saving to server
Adds virtual 'length' property to collection