Checkout 0.7.3 before testing in Travis

Be sure to revert this before merging with master.
Merge branch '0.7.3' of github.com:sympy/sympy into 0.7.3
Bump the version to 0.7.3.rc1
release/README: fix a typo
fabfile: fix the path to py3k-sympy
Copy the docs files properly
Remove and reclone sympy before release

See the README for more information how to use it.
Fix indentation problem
Allow to run "prepare" repeatedly
Add .gitignore for the release directory
Add a fab command to restart the network on the VM
Fabric: Use the current branch by default

This will generally be the release branch already.
Don't use "sympy" in the function names (what else would it be?)
Name the PDF docs using the same convention as the html docs
Make sure all files are included, and have the correct names
Add commands to check the output of the tarballs
Fix md5 checking of Python 3 tarballs
Add some comments
Factor out filenames to a helper function

I can't test this completely yet, because of the "No hosts found. Please
specify (single) host string for connection:" bug.
Use a better name for the virtualenv