Fix for 1071: Output Formatters should be invoked for writing out the response.
Issue #1141 - When Accept header or Accept-Charset header has invalid QualityFactor we throw.
Fix: Imitating the same behavior as it is in WebApi. We ignore the entire header even if one part of it is invalid.
Fix for #975 - Implementing IActionConstraint and ActionMethodSelectorAttribute

IActionConstraint follows a provider model similar to filters. The
attributes that go on actions/controllers can be simple metadata markers,
the 'real' constraint is provided by a set of configurable providers. In
general the simplest thing to do is to be both an
IActionConstraintMetadata and IActionConstraint, and then the default
provider will take care of you.

IActionConstraint now has stages based on the Order property. Each group
of constraints with the same Order will run together on the set of
actions. This process is repeated for each value of Order until we run out
of actions or run out of constraints.

The IActionConstraint interface is beefier than the equivalent in legacy
MVC. This is to support cooperative coding between sets of constraints
that know about each other. See the changes in the sample, which implement
webapi-style overloading.
[Fixes #1155] Added functional tests to cover common versioning scenarios.
Fixed up references
Removed another unneeded dependency
Use CodeBuilderContext instead of CodeGeneratorContext.
Use Microsoft.AspNet.Razor.Runtime project.
Configure host to utilize tag helper types.

- Set the appropriate GeneratedTagHelperContext properties to ensure code is generated correctly.
- Set the TagHelperDescriptorResolver.

#1165
Add writing scopes to RazorPage.

- RazorPage now has the ability to use writing scopes to control where things are written.
- This enables RazorPages to use these writing scopes with TagHelpers.  TagHelpers use them to buffer attributes that have C# contained within them and to also buffer content of TagHelpers whos ContentBehavior is Modify.
- Added RazorPage tests to validate their functionality.

#1102
Add CreateTagHelper to RazorPage.

- The CreateTagHelper method is responsible for creating and activating TagHelpers.
- Added the support for requesting the ViewContext.
- Added tests to validate the tag helper creation mechanism.

#1104
Updated to work with latest Razor changes.
Add TagHelper functional test.

- Added an end-to-end test that verifies all content behaviors and functionalities of tag helpers.
- Added some common user scenarios to verify that the system works how we expect.

#1116
Addressed code review comments.

- Also rebased on latest dev to incorporate roslyn changes.
- Modified tag helpers to work with new TagHelper format (swaped the TagHelperOutput and TagHelperContext params).
Removed unneeded comments in Startup class.
Rebased onto latest Razor code.