Lazy instatiation of Testbed to prevent SSL warning

Under Python 2.7, the production server prints warnings on every
request about a missing "urlfetch_cacerts.txt" file. The easiest
fix is lazy instantiation of the Testbed() object, as discussed at:
https://code.google.com/p/googleappengine/issues/detail?id=5578
Merge pull request #51 from awrichar/ssl_error

Lazy instatiation of Testbed to prevent SSL warning