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
added helper methods for managing SSL certificates
refactored setting verify_callback

log a message when using a custom (non-default) callback
log a warning when rewriting already existing certificate file
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
Merge pull request #69 from lslezak/sha256

Added SHA256 fingerprint support