Show full help text for -h/--help arguments

Tested on Leiningen 2.2.0.  Leiningen 2.x parses the first line of a
plugin's task docstring to produce abbreviated help messages (see "lein
help" for examples).  When a plugin's task is invoked via any of the
following commands, the task's entire docstring is printed.

* lein help <task>
* lein <task> -h
* lein <task> --help

This change moves the usage string out of show-help and into exec's
docstring.  show-help's functionality is preserved by calling into
leiningen.help/help-for to print exec's docstring to STDOUT.

Signed-off-by: Mike Neilsen <mneilsen@acm.org>