Remove of in-memory cache, and check for gzipped files. Remerge of PR #36

As requested in issue #90, I have attempted to do a remerge of pull request #36.

I have also made som minor fixes, so that it passes the tests that have been added since the original pull request.

It does fail one test however, but it's the same test that this master (original repository) currently fails as well (prior to merging of the pull request):
`✗ head must has no body » expected '' to be undefined // node-static-test.js:185`