Allow set idle timeout

Based off of https://github.com/stellar/stellar-client/pull/860.

Save idleLogoutTime in milliseconds.
Provide a dropdown with friendly labels.
Save automatically when the time is changed.

Fixes #583.

![screen shot 2014-10-02 at 1 45 34 pm](https://cloud.githubusercontent.com/assets/3108007/4498055/1f1c3512-4a75-11e4-8834-c75aaa0296c8.png)

I think prod default should be 15 minutes.