Enhance Value types to accommodate additional ways to manipulate values
Enhanced AnswerCollection so it can store multiple answers with the same name but different types. This is not common in HotDocs, but it is supported on the desktop and server. It can be necessary to support e.g. a template with a text variable "X", where the template inserts a subtemplate that has a number variable "X".  The answer file for the template therefore needs both a text value for "X" and a number value under the same name.

This means when looking up answers in an answer collection, the desired ValueType of the answer must be supplied in addition to the name.
Also added some functionality to AnswerCollections for dealing with (old) XML DTDs, as well as for providing notifications of answer changes.
Added support for passing the expected ValueType when fetching an answer for an answer file data source.
Merge pull request #3 from HotDocsCorp/master

Merging in latest SDK changes