API core URL incorrectly determined

[steveliem]

stashy/stashy/client.py

Line 73 in ae8c089

self._api_base = self._base_url + "/rest"

In this specific line the code causes a 404 error, resource not found.
This is because we have to add the extra 'core_api_path' string behind the '/rest/'.
Then we will have the actual API base URL for Bitbucket Server.

Please fix.