Display help more liberally.

Plow now displays help if you:

 * don't enter any args or commands
 * enter "help" or "usage"

It will continue to display help if you do:

    plow --help
exit with 0 on help
use return code EX_USAGE (64) from sysexits.h

Similar to other commands such as mkdir
Default environment to staging.
Update exit status to follow bash conventions.

Previously, it explicitly returned 0. As this is the default for exit it
confusing and unconventional.

Also added a note explaining the meaning and source of exit 64.
Merge pull request #7 from MarkBennett/add-help-on-usage

Display help more liberally.
Merge pull request #14 from MarkBennett/default-environment-to-staging

Default environment to staging.
Don't overwrite the .env.sample file if it exists

Previously when you run install it overwrites the file that's already there. If I install this into an app directory that already has a .env.sample I lose all the sample env vars for that app.

Similar to https://github.com/mm53bar/pave/commit/31a0ef600c5b891e5efddb87aed0568d89c94a58
Merge pull request #17 from jbabe/dont-clobber-existing-env-sample

Don't overwrite the .env.sample file if it exists
It's bad form to overwrite an environment variable that is set by the system. Plow will now use  rather than mike when connecting to remote servers to preserve the mike variable that refers to the current user.
SSH_USER is a shitty name. DEPLOYER is much better.
You can now reference recipes on the command line with or without the .sh extension
Merge pull request #18 from mm53bar/ssh_user

Stop overwriting $USER
Cleaned up environment file check
Cleaned up some of the command line options. Breaking changes.
Added some breaking changes to the param flags
Missed the semicolons
Made sure to override user even if an SSH_USER is specified in .env file
Reworked some of the arguments. Also added a check to make sure files exist in Plowfile before trying to copy them.
Made sure that the overridden user actually gets overridden