proposal for using service_name in cx_Oracle.makedsn

**The problem:**

Currently, in **Oracle** dialect, there is no way to tell sqlalchemy to connect to a database via service_name (a very common situation in Oracle RAC instances). The exisiting solutions are:

- use a full DSN in the host part of the connection string (very verbose)
- configure ``tnsnames.ora`` on server side and use that configured TNS name in your connection string (not very transferable)

**Proposal**

This PR introduces a new bool argument ``use_service_name`` (currently only via query string arg). 
It would control how to create a DSN from the [cx_Oracle.makedsn](http://cx-oracle.readthedocs.org/en/latest/module.html?highlight=makedsn#cx_Oracle.makedsn) function i.e. it would treat ``database`` part of the connection string as a ``service_name`` rather than ``sid``.

**Further development**

I would be more than happy to also update ``OracleDialect_cx_oracle.__init__`` method, write a test case and update documentation after this proposal gets an initial acceptance.