Generic HTTP cache code

[sebb <se...@gmail.com>]

There are now two scripts that include local HTTP cache implementations.

The cache code is different as they support different types:
HTML and Last-Modified
JSON and Etag

I'm hoping to merge these at some point.
And maybe allow them to work with non-text data (will probably need to
add a separate index to hold the header data which is currently stored
in the files themselves)

Note: I looked around for an existing local cache, but none seemed
entirely suitable.
Either they were too simplistic, or they required lots of dependencies.
Also the site-scan cache needs to store the URI.