Merge changes from PR #47 related to windows platform support.
Add full windows platform support including correct error checking and working regression tests and examples.
Remove need to build applications using libcoap with -DWITH_POSIX and to include coap_config.h. This applies to the examples as well.
The subdirectory "tests" now contains a growing set of unit tests for the
libcoap API. To use this, you will need CUnit (see http://cunit.sourceforge.net)
and must configure with the option --with-tests.