PLT-469 - Value schedule

Add ValueSchedule and ValueStep
Allows the representation of a value that changes in a schedule
PLT-469 - Value schedule

Update from PR
Add additional factory to ValueSchedule
More docs
PLT-469 - Value schedule

Update from PR
Rename ofAbsolute() to ofAbsoluteAmount()
Merge pull request #26 from OpenGamma/topic/PLT-469

PLT-469 - Value schedule