add default HTTP verify_callback, allow customizing it

verify_callback can be used for obtaining details about failed SSL verification,
the default implementation just logs the details
added .changes

version 0.0.15
refactored setting verify_callback

log a message when using a custom (non-default) callback
allow to exclude development and test group from being installed
Merge pull request #67 from lslezak/verify_callback

Add HTTP verify_callback
Merge pull request #68 from lslezak/ssl_cert

Helper methods for managing SSL certificates
Update .changes
added method for computing SHA256 fingerprint