check key exists and add params to get() method

firstly I'm not sure whether I should throw an exception when a key is not found or return NULL, although I'm thinking people would want an exception since it appears to be the status quo, let me know.

secondly, I think there is a simple fix to the get() problem by just allowing it to accept parameters, hurts nobody, makes the api cleaner than making people use sendRequest without good reason
This should be a separate PR. :)

`NULL` should be `null`.