Fix `mul` simplification
Remove the `for` loop inside `from_dict`
Add `mul` benchmark
Refactor `Mul::from_dict`
Merge branch 'master' into Fix_Simplification
Remove commented out code
Implement new dict_add_term
Improve readability of code
Remove mul benchmark
Remove empty lines
Use the old `Mul::from_dict` from master
Add a test for `expand`
Travis: Test Python in out of tree build

By default we should be using out of tree build, so we now test that by
default. The in tree build is only tested few times so that it works (both with
and without Python 2.7), but for all the Python versions, we just check the out
of tree build.
Improve a comment about std::atomic
Merge pull request #222 from certik/comment

Improve a comment about std::atomic
Use std::atomic for hash_
Document why hash_ and refcount_ are mutable
Merge pull request #226 from certik/comment

Document why hash_ and refcount_ are mutable
Merge branch 'master' into hash_thread

Conflicts:
	src/basic.h
Merge pull request #225 from certik/hash_thread

Use std::atomic for hash_