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 this is due to another issue, which is already documented here: #99 
`✗ head must has no body » expected '' to be undefined // node-static-test.js:185`