rename internal property constructor to klass

Since constructor already is a special property, I renamed it to klass to make the code a little bit easier to read
feature: registerFactory (naive implementation)
DRYing implementation, removing so much copy-paste
readme: Add documentation about registerFactory
Merge pull request #1 from kesla/registerFactory

Register factory
Fixed async error handling
Renames Class to Service and forced error in callback
Fixed bug when resolving async sub dependecies
Refactored for better readablity and fixed async error when missing dependency
Fixed factory returning undefined issue
Handling of resolving nothing fixes marcusberner/sandal#2
Allowed registration of undefined
Bumped version to 0.7.3
Added transient support
Removed debug console.log