Added possibility to add custom URL parameters

[petr-fiala]

viz #14

usage:

var query = Query(resourceType: SomeResource.self)
query.include("test")
query.addDescendingOrder("updatedAt")
query.addCustomURLparameter("customKey", value: "customVal")

result URL will be:
someserver.com/api/some?include=test&sort=-updatedAt&customKey=customVal