Add support for Firefox

[dsebastien]

Self explanatory :)

[dsebastien]

I just tried using Firefox 28 on Windows 8.1 64-bit and functionality wise, everything seems to work fine. I only notice an issue with the CSS applied to the Reset, Clear Everything & Save buttons.

At the time I developped the app, most CSS3 properties had to be prefixed; this has evolved now and the syntax has changed for some.

I'm going to update the stylesheet :)

[dsebastien]

Fixed the gradient, change online :)

[dsebastien]

Okay found the culprit. It's the new HTML5 input type=number that isn't correctly supported under FF :(
I suppose that they didn't agree on the implementation. The result is that the following doesn't work under firefox: <input type="number" min="1" id="duree" required ...

The 'min' attribute doesn't seem to be taken into account.

Worked around this issue by putting the duration to 1 if 0 is provided...