Add support for mapping functions

Basic support for mapping functions.

paramMapping now can take an array as value, first item of array is the string, second item is a function.

For the fields page and perPage, the function takes as arguments page and perPage.
For the field total_pages, the function takes as arguments the current value on meta, page and perPage.

It solves my issue with django tastypie backend, it might not be the best solution, but it's working and all tests pass.