Support for releases without the stable_id tables

Hi Jan, 
I just added code to fix the find_by_stable_id functions and stable_id properties in exon, gene, transcript and translation, to support the changes introduced in schema 65. I ran the test for version 60 to see if I didnt brake anything, and seems ok. I also added tests for the functions I changed calling the ensemble genes version 70. Originally, I intended to use the release from the get_info method, but I found it would complicate the version on ensemble genomes, as the release are not the same as the API version there. So I coded a function that detects if the stable_id tables are present or not, and store that as a class variable in the Base. That is reset each time a new connection is done. So, the overhead of detecting which version of the code shouldn't be noticeable. 
Fixes: #11

Cheers
Ricardo.