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.
Quote the right part of the request URL
Restrict header encoding to specific headers

In ResponseManager, do not encode all headers to unicode.
Encode ONLY the headers mentioned by the caller method.

A caller method can use the headers_to_decode and header_prefices
property-getter/setter methods to set a list of header keys and
a list of header key prefices for encoding. All keys are case
insensitive.
Add lists of expected headers in Client classes
Handle console input/output encoding
Distinguish quoted from unquoted request headers

Introduce the "headers_to_quote" and "header_prefices" properties
in RequestManager class. Modify the class so that only matching
API-specific header values are encoded and quoted.
Protect encoding-vulnerable error reporting code