Enable cpp-netlib to be build into a single library

Sometimes I find it tedious to include the various cpp-netlib libraries in my project. Thus I added the possibility to build a single library out of all cpp-netlib's separated libraries -- including uri (I'm sending a PR there too).

The configuration of the tests and examples has been adjusted to cope with the fact, that when `CPP-NETLIB_BUILD_SINGLE_LIB` is enabled, the separated libraries are not build.

Tested with GCC 4.7 and Boost 1.53.

Notes
  1. This PR includes an update for the uri submodule provided by an PR there
  2. This PR is based on my other PR #226 fixing http-client_test