variable name "window" causing issues for coverage tools

[rajarshigoswami]

File : https://github.com/wyuenho/backgrid-paginator/blob/master/backgrid-paginator.js
Method: makeHandles
Line No : 385,386

the variable window causes issues when the file is passed for coverage by blanket or istanbul
Ref issue : alex-seville/blanket#311

If it is renamed to something else, it will fix this bug