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
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
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
Added arguments for user-defined infinitesimals
Moved matching of user defined infinitesimals to classify_ode
Merge branch 'master' into liefinalintegration

Conflicts:
	sympy/solvers/tests/test_ode.py
Merge pull request #2359 from Manoj-Kumar-S/liefinalintegration

Integration of the Lie Group Solving method