add pthread condition variable implementation
Merge pull request #771 from BytesGalore/add_CV_module

pthread: Add condition variable module
core: make a thread remember its creation flags
core: Add daemon threads

Many high-level programming languages that support threading have the
concept of "daemon threads". E.g. Java and Python.

This PR adds deamon threads to RIOT. If the system is compiled to shut
down if the last thread was finished (compare #972), then a daemon
thread won't prevent that.
Use DAEMON_THREAD for daemon threads in sys