MAINT refactor fast_dot

* check for NumPy >= 1.7.2 at import time, instead of in every call;
* save a function call when newer NumPy is available;
* rewrote documentation;
* put the docstring on fast_dot instead of _fast_dot;
* remove spurious warnings for matrix-vector multiplications (some of these
  are suboptimal with older np.dot, but the good ones were triggering a
  warning as well).
ENH Upgrade to joblib 0.8.0a2
ENH: reduce memory usage and IPC overhead when fitting forests by using the new threading backend
ENH Release gil in _tree.pyx:predict
ENH Use backend="threading" for forest regression as well
DOC Update whats_new.rst to document threading backed for forests
FIX: broken sed command under OSX in joblib sync script
ENH: parallize the BaseForest.apply method with the threading backend