Use Link outside of a router

[pshrmn]

This should resolve #3889.

I added a note to the documentation listing the restrictions on this, which are:

1. to must be a string. If to is an object, then the url will be /[object%20Object].
2. If any active aware props are used, then a location prop must be included in the <Link>'s props.

[timdorr]

LGTM

[ryanflorence]

I'm actually considering not supporting all the location from props instead of context stuff. The idea was for redux support but I think ControlledRouter is better.

So, that leaves testing as the only use-case for rendering any of these components outside the context of a router. In v2 that was nice, but in v4 you can just wrap your unit test in a MemoryRouter and be done with it. So I'm going to be removing all the "prop instead of context"