Use of composer for autoload & dependencies

Hi Knp Labs!

I was searching for a nice Piwik API client and I found yours.
The code wasn't really up to date, so I tried to give it some fresh love.

I am open for any comments or suggestions.
If you merge this PR, could you please create a tag, for example "v0.1"?
Thanks for sharing this.

Cheers,
Thomas.