Some small improvements to MobileServiceCollection

A couple of improvements to the collections in the Managed PCL Client SDK:

(1) Renamed the generic parameters 'TCol' to 'TCollection' since 'Col' usually stands for column, making the type signature confusing.

(2) Added MobileServiceCollection<T> : MobileServiceCollection<T, T> and MobileServiceIncrementalLoadingCollection<T> : MobileServiceIncrementalLoadingCollection<T, T> since usually the table type and the collection type are the same.

(3) Fixed the OnExceptionOccured protected virtual member of MobileServiceIncrementalLoadingCollection to return a bool to indicate if the exception has been handled or not.