Reuse curl handle to improve performance; fixes #14

I removed all instances of `curl_close()` and turned `$ch` into a property so that the HTTP connection will stay open (using KeepAlive) throughout the lifetime of the object.

All unit tests seem to be passing, but there may be special cases not covered by the tests (e.g. what if the server drops the connection in between two requests?) so very thorough testing is recommended.