Added support for Xero accountancy software.

Added support for Xero.com accountancy software.

My CSS skills aren't great for getting the timer button nicely lined up with the other icons already on the tool bar in Xero. See attached screenshot for current layout.

![screen shot 2014-05-19 at 16 47 51](https://cloud.githubusercontent.com/assets/104929/3016344/f79987a6-df6c-11e3-9b92-45fb696115fd.png)