Create unit test framework by reusing embunit in RIOT submodule

[kb2ma]

Based on the recent addition of uint parsing for the Observe option, it really seemed like time to add unit testing. This commit adds the basic framework, and then I'll follow-on with an Observe-specific test.

This commit adds a top level tests directory for unit tests. The tests depend on reuse of the embunit framework in the RIOT submodule. The tests/embunit directory includes Makefiles to build the framework from that source.

I also added the ability to compile nanocoap source to object files in the nanocoap Makefile. This compilation is only called by the unit test Makefile as needed. I also added a section in the README to describe setup and use.

I'm sure the Makefiles can be streamlined, but hey, it's a start, and definitely worthwhile!

[kb2ma]

Two updates:

1. Added .gitignore to initial commit and squashed.
2. Added commit for Observe registration test.

[haukepetersen]

@kaspar030: what do you think about this PR? I think having unittests for nanocoap is quite helpful. Alternative could be to simply move nanocoap into RIOT and use the existing infrastructure :-)