* mode.py now defines default_mode, which is read from THEANO_DEFAULT_MODE env. var (or defaults to FAST_RUN if absent).
* added registration of DEBUG_MODE string for DebugMode (old OptCheck)
* all unittests now run with default mode. To change mode, run unittests like so:
  THEANO_DEFAULT_MODE=[FAST_COMPILE|FAST_RUN|DEBUG_MODE] nosetests
  Note: unittests specific to one mode (ie. gemm and blas) were kept in FAST_RUN only

TODO: once DEBUG_MODE is made such that it defaults to FAST_RUN after the first successfull execution of the graph, put DEBUG_MODE as the default