Thrift: Handle absence of fastbinary.so

Thrift: Handle absence of fastbinary.so

Summary:
- Wrap import of fastbinary in a try/catch, setting it to None if it fails.
- Don't try to fastbinary.encode/decode if fastbinary is None.
- Always generate thrift_spec, even if it is None.
- Don't try to fastbinary.encode/decode if thrift_spec is None.

Reviewed By: mcslee

Test Plan:
test/py/Test{Client,Server}.py
Manually delete my fastbinary.so, they worked fine.
Changed them to use Accelerated protocol.  They still worked fine because
the test falls through to the normal generated code.
Installed fastbinary.so, everything worked fine (eventually).

Revert Plan: ok

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@665236 13f79535-47bb-0310-9956-ffa450edef68