Remove plugins before copying them again.

This way plugins can rename/delete files w/o having old files lying around.
Only delete core plugins.
Zero down-time deploy and server checks

This change makes Dokku start up the new container, run a set of checks
against it, and only switch traffic over to the new containers if all
checks complete successfully. No requests are dropped during the switch
over.

To specify checks, add a CHECKS file to the root of your project
directory. This is a text file with one line per check. Empty lines and
lines starting with # are ignored.

A check is a relative URL and may be followed by expected content from
the page, for example:

  /about     Our Amazing Team

Even if you don’t use any checks, this change will prevent downtime
during switching from old to new container.

See: https://labnotes.org/zero-downtime-deploy-with-dokku/
Added man page plugin command warning

As of now, the man page does not update when plugins are installed.
Updated man page date

I forgot to change it. I even left myself a reminder!
update box to trusty
Merge pull request #628 from voronianski/master

Update Vagrant box to trusty because of raring server issues
Use PLUGIN_PATH in dokku default case.
Add CNAME option

When a repo have a CNAME file, it will use the URL on it to map the server_name on ngnix
Merge pull request #562 from assaf/zero-downtime

Zero down-time deploy and server checks
Merge pull request #657 from arthurnn/CNAME_option

Add CNAME option
move post-deploy hook

Write $DOKKU_ROOT/$APP/URL before post-deploy hook.
Revert "Add CNAME option"
Merge pull request #696 from progrium/revert-657-CNAME_option

Revert "Add CNAME option"
Make help output more consistent
Actually suppress the output from the git commands
Fixed key format check
Display help when invoking dokku with no parameter
Merge pull request #701 from jazzzz/fix-key-format

Fixed key format check
Merge pull request #703 from jazzzz/help-with-no-parameter

Display help when invoking dokku with no parameter