URL shortening speed improvement

I've made some improvements in the way ShortUrl objects are created:
-add database index for original_url field;
-use get_or_create QuerySet method;
-if exception occurs, other than URL validation (violation of unique constraint) repeat
 the process of generating shortened_url and checking for token exhaustion until object 
 is created.
 
I also perform tests usint python timeit module with PostgreSQL database createing 
10000 records.

Results:
Before changes:
using autocommit mode:    201.8245
with commit_on_success: 104.3924

After:
using autocommit mode:   146.3360 
with commit_on_success:  48.5006