Split _rt_cpan in two regular expressions

This allows finer grain of control (and quite frankly, makes it a bit 
easier to read). 

* Fix #914 by requiring RT for - and : separators (RT-1, RT:1), and no
  spaces
* Fix #914 other half by needing RT for # as well for now. If issue
  tracker is github, we still allow without prefix. RT is default
  tracker, so we cannot make broad assumptions here.
Merge pull request #916 from omega/omega/914-tighten-regexes

Split _rt_cpan in two regular expressions
Only show changes if we find the right version

If we cannot find the version of the release we are looking at in the 
parsed CPAN::Changes, we won't show anything. This should prevent some 
of the broken changes seen in issue #914
Merge pull request #917 from omega/omega/hide-wrong-release-changes

Only show changes if we find the right version
Add our own Changes parser based on CPAN::Changes

CPAN::Changes aims to provide a specification for Changes files, which
might not be exactly what MetaCPAN wants. For now we steal most of the
CPAN::Changes framework, but replace the parser itself with one that
is a bit more relaxed.
Switch to use our own Changes parser
Support nested Changes entries in HTML

This is a bit of a hack, based on our own Changes parser, that looks for
`-` and then interprets that as a nested change. Will need adjustments later
Merge pull request #902 from omega/omega/dbic-changes-display

Fix formatting for nested Changes files (#897)