Proper names for providers

RequestTimeProvider has nothing to do with RequestTime. It just stores argument with some light parsing (which is bit magic but out of scope of this pull), therefore renamed to ConstantProvider.
StandardProvider name references to undefined standard. I suggest renaming it to Current(Time)Provider or NowProvider (so it reads Kdyby **Clock** Providers **Now** Provider).

I can squash it if you don't use [github](https://github.com/github/hub) [cli](https://github.com/schacon/git-pulls) tools already.