Added availability to use custom WebService name

See merge request rhab/PyOTRS!10

to build the url. Fix tests and added one to test the change of webservice_path

fix #10 bug - hardcoded `TicketGetList` in Client.ticket_get_by_id(). 
Instead hardcode it to `TicketGet` (which should always be available). 
This also makes it necessary to pass `ticket_id` to _send_request(). 
Also Minor improvements in more pythonic way of doing things. 
Many thanks to @dmantis

See merge request !6

add option to get for html body as attachment

See merge request !5