README: update build-passing img url

Cause there is a newer url format now.
Readme: heavily update readme

Fix some falsities, include most resent help, move some links to bottom of the
document.
Help: improve descriptions a bit
Make middleware function public

Instead of passing in a function to plugin.init(),
just move the middleware registering function to a public API.

Not sure why I didn't do that in the first place, it's a lot more
consistent with the rest of the functionality (addBuild,
consumeBuild, ...).
injectBuild: move port param into options hash

No need for an additional parameter.
Post-receive hook: support multiple lines of input

Git will send one line for each ref being updated, we need to be able to handle
that.
Merge readme-update-it (#34) into master

Readme: heavily update readme

Fix some falsities, include most resent help, move some links to
bottom of the document.  Also, improve the Help descriptions a
bit.
Merge register-middleware-function (#35) into master

Make middleware function public

Instead of passing in a function to plugin.init(),
just move the middleware registering function to a public API.

Not sure why I didn't do that in the first place, it's a lot more
consistent with the rest of the functionality (addBuild,
consumeBuild, ...).
Merge post-receive-hook-allow-multiple (#36) into master

Post-receive hook: support multiple lines of input

Git will send one line for each ref being updated, we need to be able
to handle that.