adding a clear button to elm

Clears all the logs that have been stored thus far.
![elmlogger8](https://cloud.githubusercontent.com/assets/2818455/5305043/d6300e46-7bac-11e4-9f5b-10e8c5e21dc6.png)

I think this sequence of `if`s would be a bit easier to read as a sequence of `if-else-if-else`s