[base] DateTime: fixes compare() in order to get < <= > >= operator overloads work
[base] Signal: added clear() to remove all handlers at once
[http] HttpConnection: fixes connection reusage.

We did not properly reset the input buffer which caused
further http connections to abort as soon as one caused a 400 Bad Request.
[http] HttpConnection, HttpRequest: code design cleanups.

Merged 2 state enums into 1 more precise one, and so fixed some bug I
ran into.

HttpRequest now supports onPostProcess and onRequestDone locally,
something we will need in the upcoming cache layer for the director plugin.
[plugins] status: adapted to changes in HTTP API
adding TBB (Threading Bilding Blocks) library to the list of dependencies.
[plugins] director: code cleanup wrt FastCgi health monitor
[plugins] director: added experimental support for response object caching.
fixes #72 - compilation error due to missing Compiler support @ GCC on Ubuntu (12.04)
[plugins] director: more work towards concurrent thread-safety on backend clusters.