add hint for missing package on Debian Wheezy

the python-augeas itself does wrongly not depend on libpython2.7, so if
libpython2.7 is not yet installed, an 'import augeas' in python fails.
This could easily be solved by manually installing the missing package.
Merge pull request #5 from zauberpony/master

add hint for missing package on Debian Wheezy