Test for CURLOPT_POSTFIELDS lifecycle.

libcurl does not copy POSTFIELDS data, therefore pycurl must keep
that data around until the request finishes.
The memory is no longer allocated (c031f4591b99a7ed3c05e8ef3e11cb7814014df5)
Keep a reference to the object used for CURLOPT_POSTFIELDS.

Fixes #34
Update changelog
Break out postfields object into its own memory scope
Zap for safety
Merge pull request #35 from p-push/string-options-copies

Increment reference to CURLOPT_POSTFIELDS object