refactored "parseMarkup" logic on viewEngine

Based on this Issue: https://github.com/BlueSpire/Durandal/issues/141

* Renamed "parseMarkup" to "processMarkup", since it did more than just
parse.
* Renamed "parseMarkupCore" to "parseMarkup" since that really did just
parse.
* Exposed "parseMarkup" as public as well to make it overridable.
* Updated the unit tests to reflect this and test the new public
"parseMarkup" overriding logic