[#732] Attribute Routing: Implement `~/` for overriding a prefix

This change allows a user to override a route prefix set using
[Route("...")] on the controller by providing a route template
on the action that starts with "~/" or "/". For example,
[HttpGet("~/...")] or [HttpGet("/...")]

If the user specifies a template in [Route] that starts with "~/"
or "/", we will just strip the prefix from the template and use
the remaining part of the template.

The reason to do this is that there's a reasonable extensibility
scenario where a user can implement a global prefix for routes as
a convention (using IReflectedApplicationModelConvention), and use
~/ to escape that prefix (just like we support with action-level routes).
Cache file info access in viewengine

Move compilation and VirtualPathViewFactory to be singletons
And cache access to files.

The cache time is controlled by MVC options.
The cache is implemented in the ExpiringFileInfoCache.cs
IUrlHelper and IViewComponentHelper need to be activated

* Removing Init method and using [Activate] to activate IUrlHelper.
* Moving IViewComponentHelper to default injected properties
* Adding functional tests to verify these properties are injected \
  activated

Fixes #789
Fix for issue #610 - Ignore internal and nested types

This change exludes internal and nested types from being treated as
controllers. This is consistent with MVC5's behavior.

DefaultActionSelectionConventions was primarily tested through running
action selection. I wanted to also test the methods with substantial logic
in this class, so I moved a spate of a classes from private classes inside of the
integration tests to public classes so they could be shared. I also added
tests to fill gaps in DefaultActionSelectionConventions, which is the vast
vast majority of this change.
Reacting to System.Security.Cryptograph.Encryption package rename
Move ValueProviders to MvcOptions
Fixes #778
Separating view execution and Razor behavior

* Introducing RazorPage and RazorPageOfT that represent the Razor
  execution aspect of view execution. Moving view execution hierarchy behavior
  (Layout, partial views etc) into a separate RazorView type.

* Renaming IVirtualPathViewFactory to IRazorPageFactory,
  IRazorViewActivator to IRazorPageActivator

* Renaming VirtualPathViewFactor to FileBasedPageFactory to
  correctly reflect what it does.

Fixes #814
Adding RazorView to Microsoft.AspNet.Mvc.Razor

RazorView was part of the previous commit but was separated to make it
easier to see the diff in RazorPage that was formerly named RazorView

Adding IRazorPage and changes per code review comments
Http Abstractions for parsing Http headers needed for conneg
Adding OutputFormatter base class
Responding to comments, adding summary headers.
Responding to comments and changing the interface to take in a formatter context.
fixing test
Responding to comments
Adding OutputFormatter base class
Responding to comments and changing the interface to take in a formatter context.
Responding to comments
Adding MvcOptions.OutputFormatters property.
Moving to OutputFormatterDescriptor
Responding to comments and changing the interface to take in a formatter context.