add 'validate' option to disable or enable hostname verification

[perfectspr]

fix #295

[timarmstrong]

I think it would be clearer if the parameter name made it clear that it was verifying the SSL certificate. I looked at other dbapi drivers and everyone calls this parameter something different. ssl_verify_cert is MySql's choice and is unambiguous, so I think is a good choice - https://dev.mysql.com/doc/connector-python/en/connector-python-connectargs.html. Could you rename the "validate" argument to "ssl_verify_cert"?

[perfectspr]

renamed "validate" to "ssl_verify_cert"