Cleanup namespaces and Task.FromResult
Modifying test project.json to allow building on Mono
[Fixes #429] FileResult

1) Implemented FilePathResult to efficiently return files from disk.
2) Implemented FileStreamResult to return content from a stream.
3) Implemented FileContentResult to return content from a byte array.
Changes to support PageExecutionContext in Razor
Fixes #1083
Changes to make EnableInstrumentation conditionally enabled
Calculate `ModelMetadata.DataTypeName` based on metadata
- helps MVC helpers like `@Html.EditorFor()` select the correct template
- #933

nit: add XML doc for `DataTypeName` in base `ModelMetadata`
Correct minor problem in `CachedDataAnnotationsModelMetadata.ComputeDisplayFormatString()`
- only affects an extreme corner case: user sets `metadata.EditFormatString` then reads
  `metadata.DisplayFormatString`
- an extreme case because `EditFormatString` is normally set only when
  `DisplayFormatString` is set and, if set, it's to the same value
- happened to see this while updating `CachedDataAnnotationsModelMetadata` for this PR

nit: an -> a in an adjacent XML comment in `CachedDataAnnotationsModelMetadata`
React to roslyn changes
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
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).