Integrate fsharp-doc

I've reset to the last known safe point before you had a go integrating `fsharp-doc` (34fb951) and put my changes on a topic branch.

I've added fsharp-doc.el and its test suite. This has a dependency on my `namespaces` package for elisp - if you don't want that, let me know and I'll refactor it.

I also made some changes to the pos-tip stuff. Any use of pos-tip fails in OS X Terminal, so I added some tests to see whether its available, falling back to the minibuffer if not. There's a var, `ac-fsharp-use-pos-tip`, to give users explicit control.