Add docopt library
Made snapshot.set() method operate relative to CWD

Before it was relative to ~/.rab
Implemented docopt

Usage:
    rab snapshot create PATH...
    rab snapshot delete PATH...
    rab snapshot <name> add PATH...
    rab snapshot <name> reconstruct <file> PATH
    rab blocks delete
    rab (--version | -h | --help)

Options:
    -h --help  Show this help message and quit.
    --version  Show version.