Added category filter to products
Merge pull request #19 from mechatama/products-category-filter

Added category filter to products
Add filter documentation for various resources.

Trying to bring some consistency to the documentation with regards to filtering resources.

 * Where possible it is now mentioned that GET requests will return a maximum of 50 resources when no filter has been specified.
 * limit and page parameters documented for resources that support pagination.
 * min_id and max_id parameters added for those resources that support them.

Note that some resources still require their filter documenation to be updated.
Merge pull request #21 from davidtsadler/filters

Add filter documentation for various resources.