Add a landing page

I find a little annoying that peperone starts a pomodoro when i visit the homepage, IMHO there should be a simple landing page that tells what it is and have some big buttons to start some timers.

But I am not sure about what should be in the homepage.. I think in the future will be a blank slate ( https://gettingreal.37signals.com/ch09_The_Blank_Slate.php ), but for now maybe we don't need much more than this...

The homepage with this pull request become like this:
![peperone-landing](https://cloud.githubusercontent.com/assets/5879/3870434/eaad3896-20ce-11e4-9bf5-8a759aec572d.png)