Installing web2py from trunk

TODO: Check this with web2py peeps

$ git clone --recursive https://github.com/web2py/web2py.git

# See what the current stable version is
$ python web2py.py --version
Version 2.12.3-stable+timestamp.2015.08.18.19.14.07

# Get the most recent tag
$ git describe --abbrev=0 --tags
R-2.13.2

# Check out that latest version
$ git checkout R-2.13.2
M    gluon/packages/dal
Note: checking out 'R-2.13.2'.
You are in 'detached HEAD' state [etc.]