Make `flocker-deploy` more universal

I'm starting to play with Flocker for the first time. So just thought I'd offer some perspective from fresh eyes. Note that I'm only vaguely aware of other factors that I'm not taking into consideration, eg; #259, #359

Two things struck me about the [current installation docs for the command client](http://doc-dev.clusterhq.com/installation.html);
* The implication that the client can only be installed on Ubuntu. I can't believe this is the case. And is surely off-putting to potential users, when I suspect the client will work perfectly well on Windows even.
* The manipulation of a user's `$PATH`. I consider this bad etiquette and should be avoided at all costs.

So regarding #359. Can Pip's package index not be used for the `flocker-cli` component at least? It has download counts and HTTPS. And then RPMs/debs for the `flocker-node` component? 

I've provided a suggestion `contrib/flocker-deploy-dev.sh` script that overcomes the need for altering `$PATH`. I'm assuming that at some point there will be an official means of installing the client which will place the binary in the system path. Therefore any other means of running the client needs to have another name to avoid conflict and also needs to offer something different, which is why I've made it run the live code from the repo, rather than the setuptools-installed flocker lib version of flocker.

BTW, how do you recommend developing against the command line? Do you use something similar to a `fl-dev` script? Or are you working in a pure TDD style?
This seems like the core of the proposal, and arguably it's worse than manipulating `$PATH`. Ad-hoc installation of stuff as root is a bad idea, that's what package managers are for. I don't want our instructions resulting in people leaving random files in their core OS that might break when they upgrade our software. I'd rather leave the  `$PATH` instructions in until we have real Ubuntu (and Homebrew?) packages available, especially since they're optional.

How about just changing this PR to simply update the prose to note that changing `$PATH` is optional and one can just use the absolute path?