Use font-lock instead of jit-lock.

This allows us to get rid of the with-silent-modifications compatibility
implementation and unpropertizing hacks.

No performance difference unless somebody intentionally misconfigures
Emacs, because font-lock uses jit-lock by default.
Highlight delimiters even in already highlighted text.

Fixes #10.
Remove the changelog.

It's outdated, and the source file is not the right place for it.
If somebody needs it for historical reasons, it's still there in
older revisions.
Clean up comments and whitespace.
Rename rainbow-delimiters-keywords.
Simplify the logic in rainbow-delimiters-propertize a little...

...by using the delimiter position returned by match-beginning instead of
moving the point to the delimiter.
Remove unneeded save-excursion.
Fix a typo in a comment.