Add TagHelperDescriptorResolver implementation.

- Added a basic TagHelperDescriptor creation mechanism to the TagHelperDescriptorResolver.
- Added an ITagHelperTypeResolver to be responsible for determining valid TagHelper types.

#99
Add tests to verify TagHelperDescriptorResolver functionality.

#99
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
Add TagHelperScopeManager for runtime.

- Also added an interface ITagHelperScopeManager so we can activate the type via DI.

#155
Test TagHelperScopeManager functionality.

#155