Add TagHelperTypeResolver.

- The TagHelperTypeResolver is responsible for determining the format of lookup text's used throughout the tag helper system.  By default it handles the following formats:
"assemblyName"
"assemblyName, #.#.#.#"
"assemblyName, #.#.#.#, specificType"
- It also restricts what types are considered TagHelpers.  In this implementation we only accept public, non-nested, non-abstract, non-generic TagHelpers.
- Added tests to validate the expected functionality.

#158