Temporary fixed bug in function_sum
Made the following changes
1] Integrated the lie group solver with dsolve using ode_lie_group
   hint.
2] Replaced int with S(int)
3] Corrected classify_ode test failures
Made the following changes
1] Added a few tests
2] Better algo to eliminate arbitrary functions from output
   of pdsolve
First shot at Theano on Travis.
Took out the comments in the yaml file.
Switched to --no-dep install of Theano.
Trying manual Theano install.
Simpler Theano download.
One more try.
Forgot semi-colon.
Made the following changes
1] Changed the algorithm so that even if solve or any other function
raises Error, another heuristic is used
2] Removed duplicated test
Marked test_BlockMatrix_Inverse_execution with @XFAIL.
Imported XFAIL.
Made the following changes
1] Improved documentation
2] Added doctests
3] Return solution for integrate as it is, if it cannot be solved
   for y
Trying SKIP.
Added arguments for user-defined infinitesimals
map() -> list(map())
Tests pass now with one more list(map()).
Made separate builds for Theano.
Only test theano code if TEST_THEANO is true.