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.
Merge branch 'sigv4-testsuite' of git://github.com/jamesls/botocore into jamesls-sigv4-testsuite