Make TagHelperOutput.Attributes case insensitive.

- Changed the Attributes dictionary to utilize a StringComparer.OrdinalIgnoreCase comparer.
- Changed TagHelperExecutionContext to ignore attribute casing.
- Updated the AllAttributes and HtmlAttributes dictionaries to ignore attribute casing.
- Added tests to validate their new behavior.

#186
Add @removetaghelper directive.

- Added parsing recognition of the @removetaghelper directive.
- Added TagHelperDescriptor handling: @removetaghelper will cause the system to ignore TagHelpers that are added via @addtaghelper.
- Added Chunk generation, this involved building a CodeGenerator (soon to be named ChunkGenerator) that pulled out the stringified @removetaghelper lookup text which it then uses to construct a RemoveTagHelperChunk.
- Modified CodeVisitors to now understand RemoveTagHelperChunk's.
- Added code generation, this involved creating a dummy string in order to give the @removetaghelper "..." coloring.

#112
Add tests to validate @removetaghelper functionality.

- Added utility methods to construct valid SyntaxTreeNodes that represent the @removetaghelper directive.
- Added parse level unit tests to validate the @removetaghelper generates an accurate SyntaxTreeNode.
- Added parse level unit tests to validate the @removetaghelper throws with bad formats.
- Added TagHelperRegistration unit tests to validate the AddOrRemoveTagHelperCodeGenerators are understood and affect the descriptors found.
- Added Designtime mapping tests to validate correct source mappings are made to ensure proper coloring and lack-of C# intellisense.
- Added end-to-end tests to validate @removetaghelper can essentially disable TagHelpers on a page.

#112
Added description in project.json for all projects
Merge branch 'release' into dev
Add DesignTime code generation support for TagHelpers.

- We now generate a subset of the TagHelper runtime code during DesignTime. This enables users to see errors in the editor
- Added tests to validate design time code generation.
- Refactored runtime code generation tests (we now use a lot of their infrastructure for the design time pieces).

#208
Addressed code review comments.