Update and sort .gitignore

Ignore the spec_bin temporary files.  Remove the static presentation
ignores for the website, since they are just linked to now.  Remove
the .rvmrc ignore, since Sequel doesn't use one.

I'm considering removal of the 3 *. entries, but I'll leave them in
for now.
Handle spec_*cov tasks without a spec/rcov.opts file

The file currently exists in source control, but with some changes
it may not, so make sure it runs without the file.
Only include specific file extensions in gemspec

The previous gemspec was pretty loose, and I could see unexpected
files slipping in by accident.  Change things so that only
expected file extensions appear in the gem.
Note exception to general code order suggestion
Fix inconsistent indentation