[install] improve distribution detection
[install] fix python version to 2.7 for now
[install] make install recipe configurable
[install] add --only-deps option
[install] print summary before installation
[install] set PYTHONPATH when using --only-deps
[install] add --without-system-packages option
[install] add --virtualenv-dir option
[install] be more verbose
[install] add help on using the new virtualenv