Update backgrid-paginator.js

Fixed goBackFirstOnSort, it was not possible to set this property to false
If the option goBackFirstOnSort is defined it should overrule the default option

if options.goBackFirstOnSort is not defined the default option must be chosen (this.goBackFirstOnSort)
undefined check
Merge pull request #8 from jurgenkalverboer/master

Update backgrid-paginator.js, Fixed goBackFirstOnSort