Reconfigured feeds tamper feature and new plugin for simple date math

Data integrity is maintained through import with this new configuration.

Debugging traced down the issue to one particular feeds tamper item. 
xpathparser:17 -> Section Information: Date Range: Start 

Two operations were being performed before. 
1. Convert to unix timestamp
2. Subtract an integer

For some reason the combination of these two caused issues with the data. 

Solution was to create a simple datemath plugin that will allow users to add or subtract time in simple english using PHP's strtotime function.