add support for client SSL certificate and key
check --ssl-cert and --ssl-key to be files
rename existing_file to readable_file_arg and move to input
rename -ssl-cert and --ssl-key to --cert and --certkey
document --cert and --certkey
added --cert to CHANGELOG and matleh to AUTHORS
Merge pull request #197 from matleh/master

add support for client SSL certificate and key
Fixed ZeroDivisionError in download summary.

Closes #202
Removed Bitdeli badge.