Release: don't build Python 3 tarball

Just build one tar.gz, it should work with all Pythons.
Use print functions in fabfile.py
Re-enable fabvenv stuff
Use @task in the fabfile

This fixes issue 4052.
Make it more clear that fabvenv is required
Use the correct url for fabvenv
Allow to test the release on your fork

This is untested.
Update tests list in setup.py

The liealgebra tests were missing.
Fix a little issue I noticed in the install guide

The ipythonprinting extension is deprecated in favor of init_printing().
Fix the release fabfile for the single Python 2/3 codebase

I believe it works (it seems to work off a branch on my fork).

Note, to test this, you'll need to comment out the mailmap_update line in the
release() function as the .mailmap file hasn't been updated yet.
Add note about tags if you test the release against your fork
Update AUTHORS and .mailmap so that mailmap_update.py passes
Merge branch 'master' into release_update

Conflicts:
	.mailmap
Add some people to .mailmap
Add Harsh Gupta to the AUTHORS/aboutus. Welcome to SymPy!
Retroactively add Jeremy to the AUTHORS/aboutus. Welcome to SymPy!
(Retroactively) add Lars Buitinck to the AUTHORS/aboutus. Welcome to SymPy!
(Retroactively) add Pablo Puente to the AUTHORS/aboutus. Welcome to SymPy!
(Retroactively) add Alkiviadis G. Akritas to the AUTHORS/aboutus. Welcome to SymPy!
Merge pull request #2600 from asmeurer/release_update

Release update