Cancelling requests, implements issue #21
removed debug output, help file with useful call for [timeout( and [cancel(.
Correctly cancelling curl requests (see http://curl.haxx.se/docs/faq.html#How_do_I_stop_an_ongoing_transfe).
Further changes for cancelling requests.

- post()  message
- updated help
- init libcurl only once.
- use pd_error() where applicable
Updates for next release:

- version number increased to 0.13.0
- Makefile from library template 1.0.14
- removed obsolete examples
- switched to semantic versioning
- multiarch compilation
- changelog
Merge branch 'master' into json-c-0.11