Fix: response.status_code is an int not a string

Also added the response reason in the error message.
Updated client to reuse auth cookie
Support a user-supplied Requests Session
helpers.py: Ensure parameters are not overwritten when paging

When subsequent pages are requested, any request parameters were being overwritten. Instead, leave the parameters as is, and simply add in the start parameter in addition.
Branches are a paginated resource, so they should be exposed like one