Use docopt instead of optparse

Although docopt is a non-standard library, I just think that it's so
much nicer than optparse/argparse to justify this extra dependency.
Simplify usage line

The line does not contain information about mutually exclusive arguments
anymore but is much easier to read.
Unindent output of --help

The __doc__ inside the _EntryPoint subclasses contains uniform
indentation that should be cleaned up before printing it to the user.
Rename CLI options and add negations for switches

Resolve #36.
Update MAN page
Add udiskie-unmount CLI switches --lock, --force
Add footer for --help
Merge branch 'docopt'