Changed Schedule.Id from int to string

Schedule.Id was typed as an int which is inconsistent with expected
values from our SDK, implementation of other types, and expected values
for some methods. I have updated it as a string and updated the unit
tests to work with this.
Updated RestClient to accept all success statuses

CTCT returns 200, 201, 202, and 204 as success messages depending on
endpoint. RestClient class will now accept all of these as a successful
response.