Change ReportProgress behavior to be consistent with documentation

This addresses two separate inconsistencies in the behavior of ReportProgress relative to other game services platforms:

1. ReportProgress is documented as receiving a ``progress`` parameter between 0.0 to 100.0.  The code was instead assuming a value in the range [0.0, 1.0].  Using a value of 0 to 100 is consistent with [Unity's Social.ReportProgress](http://docs.unity3d.com/ScriptReference/Social.ReportProgress.html).

2. Specifying any ``progress`` value greater than 0 would unlock the achievement.  This requires the developer to query the achievement ahead of time to prevent erroneously unlocking a non-incremental achievement.  Restricting unlocks to ``progress`` values >= 100 allows a developer to treat achievements as incremental and expect non-incremental achievements to only unlock when the player has completed all steps.  It's then only the app administrator's responsibility to determine if an achievement is better suited to be incremental or non-incremental, and can change the type in the developer console without changing the code.