Add sigv4 test suite

AWS provides a test suite for sigv4 that includes sample requests and the
expected signatures.  I've integrated that into the test suite for botocore.

I had to update the auth module to accomodate some of the sigv4 test cases,
the two big changes being to support multiple headers with the same key.

There were also tests that include query params with duplicate keys
(e.g. 'foo=bar&foo=baz'), but given that no AWS services support this, and no
SDKs support this, I'm ignoring those tests for the time being.

Also, for python2.x, the HTTP parser didn't support parsing header values
with duplicate keys so those tests are ignored on python2.x.

Verified that all unit tests pass in py26,py27,py32,py33.