Reduce dependency stack of rewrite map script to prevent hangs

Ensure every value in the $sites array is a genuine site dir and not an alias.
For the real dir c, this changes ($sites['a']='b'; $sites['b']='c';)
to ($sites['a']='c'; $sites['b']='c';) to ensure continued operation if alias b is deleted.

This moves the logic for cache misses to a separate script,
rewrite_miss.php, which will process the rewrite and return
the correct route. The rewrite_miss.php script includes a
time limit to prevent hang-ups from occurring.
Closes #424

[gh-387] Pull Request into develop.

Issue 124

[gh-124] Add a input filter that allows the embedding of content from nodes inside of nodes by using tags.
[gh-124] Add support for the <!--#include virtual="some/path"--> SSI directive as a filter.

Issue 402 Pull Request

Issue 396 Pull Request