Add assign function.
Fix tests timing and return value in mapper.

Glom the assign expression before the assert statement, otherwise
from_numpy gets executed after the ndarray has already been changed
(rendering the test useless). To fix this, the tests glom the
RegionMapExpr first, before the corresponding ndarray has been updated.

Because of this bug, the assign mapper bug wasn't being detected. The
assign mapper should return value, not tile. Assign mapper was kept in a
separate function (instead of a lambda expression) so debugging would be
made easier (and in case a better implementation of the mapper is
found).
Merge pull request #176 from MaggieQi/master

fix nosetest std bug, try to use simple implementation of std
Merge pull request #175 from rgardner/add-assign-operator

Add assign operator
Merge branch 'master' of github.com:spartan-array/spartan
fix region_map fn_kw containing Expr error