Restore various FromContext functions

[kevinconway]

Set the request context to the one generated for a chain to ensure
calls to FromContext and FromRequest return the same results.

[kevinconway]

Ref: #15

[kevinconway]

Worth a note, I'm intentionally not accounting for the backwards compat for < 1.7 with this PR because it's only intended as a conversation piece. I imagine, if you want this feature, that you'd want to use a similar go1.7 build tag split between two files like in the other x-libs.

[rs]

<1.7 compat is not optional otherwise this will break all existing, not yet migrated installations like yours, which is not friendly :)

[kevinconway]

@rs Yes, that was intentional. I originally didn't think of a way to introduce the change without shifting around large amounts of code so I was going to let you do it. However, I believe I was able to isolate the one New function into a pre and post go1.7 files with build tags. I believe this approach is consistent with your other libraries.