Trigger error callbacks asynchronously

This allows one to, e.g., yield the current fiber after making the
request and resume it in a callback.
Ensure uppercase request method for Curb

Curb's `http` method takes its argument and sets it as the value for
libcurl's `CURLOPT_CUSTOMREQUEST`. Apparently this *needs* to be
uppercase.
Merge pull request #305 from elliterate/async_error_callbacks

Trigger error callbacks asynchronously
Merge pull request #306 from elliterate/fix_curb_tests

Ensure uppercase request method for Curb