Add "service_type" member var in Client classes

Refs: #2

The "service_type" of an API client is related to the endpoint URL
used to initialize the client and should match the "type" attribute
of the corresponding endpoint.
Add method "get_endpoint_url" in AstakosClient

Refs: #2

In kamaki, AstakosClient is imported from the astakosclient
package, and then it is wrapped so that some convenienve methods
can be added.
Use "get_endpoint_url" in client instantiations

Refs: #2
Update docs and Changelog for get_endpoint_url

Closes #2
Merge pull request #7 from saxtouri/feature-service-endpoint

Introduce the 'get_endpoint_url' method (identity client wrapper) and use it in client initialization.