Linked back/next buttons enabled state to the state of the history. When at the beginning of the history, the back button will be disabled. When at the end of the history, the next button will be disabled.

Also added three keyboard shortcuts that are pretty standard in browsers:
Alt+Left Arrow: Back
Alt+Right Arrow: Forward
Alt+Home: Home
Merge pull request #13 from joeattardi/historyButtons

Linked back/next buttons enabled state to the state of the history, added keyboard shortcuts for navigation