Assertion if incrementProperty given non-numeric value

Includes fixes for incrementProperty and decrementProperty.

Ember.assert in incrementProperty and decrementProperty discourages any attempt to increment/decrement by a non-numeric value or Infinity.

Retains support for 0 increment

Added tests for NaN, non-numeric string, and Infinity
Merge pull request #3040 from realityendshere/increment-property-nan

Fix incrementProperty/decrementProperty to handle NaN