Documentation

* `gh-pages` branch added. JavaDocs preview is available here http://piwik.github.io/piwik-sdk-android/
* Readme.md updated
* Wiki page [updated](https://github.com/piwik/piwik-sdk-android/wiki/Design-document/_compare/1ad8b9ef2b33d1b7119a4217ab0b86eee406abc2...f1fc2e9be74e5dad57341fd08ef5d34b2be32dfe)

Refs #11 
An activity represents a single screen with a user interface and I believe every android devs should know this term.