fixes .attr('data-xthreshold') and .attr('data-ythreshold') for jquery 1...

Hi,

First of all thanks for writing the the plugin.  I was in the process of upgrading some pages from jquery 1.8.3 up to 1.9.1 and 2.0.0 when i noticed that the migrate plugin for the latest versions was warning me that mobile-events was using .attr which is no longer supported.  I've tweaked the non min and min files to replace 

.attr('data-xthreshold')  and .attr('data-ythreshold') 

to use their data counterparts as follows

.data('xthreshold') and .data('ythreshold')

so the plugin should be good for a good while longer now.

Thanks,
Phil.