Added support for box-sizing border-box

Added a fix for the width-calculation in case of 'box-sizing' 'border-box'. (padding doesn't count on top of the width in this case, so I set the padding to 0)

Would resolve these two issues:
https://github.com/vitch/jScrollPane/issues/27
https://github.com/vitch/jScrollPane/issues/180
Merge pull request #246 from pieht/patch-1

Added support for box-sizing border-box