switch-to-relevant buffer

Hi I added this function 'nrepl-switch-to-relevant-buffer'. It switches the user to the right nrepl buffer based on the current buffer. So if you have 2 nrepls open for project x and project y, if you open a project x file and run the function (key mapped of course) then you land on the right nrepl buffer. I couldn't see anything that caters for this in nrepl.el hence the pull request.

TBH I'm new to elisp so you might want to make sure it looks sane. Unfortunately since I moved this fn into nrepl.el, I can't seem to use it without eval'ling the nrepl.el in emacs. Not sure why this is? I get:

Symbol's function definition is void: nrepl-switch-to-relevant-buffer. I've tried byte-recompiling my emacs dir to no avail. 

Not sure what the issue is.