Add AssemblyName to TagHelperDescriptor.

- TagHelperDescriptor's need a way to showcase where they came from so tooling can reverse look them up.
- Update tests to specify TagHelperDescriptor assembly names.

#214
Change TagHelperAttributeDescriptor to not depend on PropertyInfo.

- Right now the only information that is used from the PropertyInfo is the PropertyName and the PropertyType, therefore changed the PropertyInfo property on TagHelperAttributeDescriptor to be AttributeTypeName since we already had an accessor for AttributePropertyName.
- Modified test comparers to validate type names.

#214
Add extensibility point to resolve descriptors in assembly.

- Modified our default TagHelperDescriptorResolver to provide a protected virtual method that it uses under the covers to resolve descriptors for an entire assembly. This will enable tooling to cache descriptors on a per-assembly basis.

#214
Addressed code review comments.