Remove plugins before copying them again.

This way plugins can rename/delete files w/o having old files lying around.
Only delete core plugins.
Use PLUGIN_PATH in dokku default case.
contrib: add dokku client shell script

This is adapted from the one I use in my shell profile scripts. This
kinda-sorta-maybe acts like the `heroku` or `deis` commands and does
some clever reorganization of data before sending it to the server.

To use the app name generator, install the `elfs` rock from Moonrocks.
port 'wait_to_retire' from broadly/dokku
Actually suppress the output from the git commands
Merge remote-tracking branch 'upstream/master' into mh-merge-upstream
Merge pull request #1 from expa/mh-merge-upstream

merge from upstream
Update vagrant installation instructions

Avoids 
```
Permission denied (publickey).
fatal: The remote end hung up unexpectedly
```
that can occur on certain Vagrant images. Since we need readonly anyway, might as well use HTTPS instead of SSH.
replace gitreceive by sshcommand in example url
fix dokku development blog link

http://progrium.com/dokku doesn't exist anymore. The blog is really at http://progrium.com/blog, but http://progrium.com is by default on the blog, so let's use this link.
Merge pull request #748 from vincentfretin/patch-2

fix dokku development blog link
The app certificates directory was renamed to tls in e2135a2c5f599184ce1f40beb21fccb9a0f8b8de
Remove unintended phrase repetition in installation.md.
Merge pull request #750 from th4t/master

Remove unintended phrase repetition in installation.md
Merge pull request #746 from vincentfretin/patch-1

replace gitreceive by sshcommand in example url
Merge pull request #749 from vincentfretin/master

Fix app certificate directory in backup-import/export
Merge pull request #718 from Coaxial/patch-1

Update vagrant installation instructions
Release 0.3.0
Use command instead of which for apt-get existential check in bootstrap.sh