cscope: don't match a token proceeded by an alphanum

Otherwise a line like "something.extend.foo; end" gets all mangled (because we
match the 'end' too early) and cscope barfs.
cscope: hide nested functions

Based on my experimentation (see #44 and #46) cscope simply can't handle nested
functions. I've tried a couple of different ways of handling this, and all of
them cause cscope to barf in one legitimate case or another.

So, when exporting, track how deeply our functions are nested, and hide (don't
tag) any nested function definitions/ends. This loses us the definitions of
those functions, sadly, but it's the best I've been able to come up with.
Starscope's own DB still tracks them properly, of course.

Fixes #46
Merge pull request #47 from eapache/more-cscope-fixes

More cscope fixes