Add TagHelperDescriptorResolver.

- This involved also adding a TagHelperTypeResolver and TagHelperDescriptorFactory.
- 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"
"specificType, assemblyName"
- It also restricts what types are considered TagHelpers.  In this implementation we only accept public, non-nested, non-abstract, non-generic TagHelpers.
- The TagHelperDescriptorFactory is responsible for converting a Type to a TagHelperDescriptor.
- Added tests to validate TagHelperDescriptorResolver, TagHelperTypeResolver and TagHelperDescriptorFactory.

#99
#158
Add tests to validate TagHelperDescriptorResolver.

- Tested the descriptor resolver and its underlying dependencies (TagHelperTypeResolver and TagHelperDescriptorFactory).

#99
#158
Add TagHelperRunner for TagHelper runtime.

- This involved adding the following core classes: TagHelper, TagHelperExecutionContext, TagHelperOutput, TagHelperContext.  All of which aid in running TagHelpers.

#154
Test TagHelperRunner, TagHelperOutput and TagHelperExecutionContext.

#154
Addressed code review comments.