Fixes #255 -- Add verification methods to the Connection object

[pablosole]

This PR adds the methods set_verify, set_verify_depth, get_verify_mode, and get_verify_depth to the Connection object.

While Connection objects inherit these values from the Context used to create it, they can be changed afterward. This is especially useful when using SNI to require different verification flags depending on the server name (ie. require client certs in some SNIs but not on others).

Fixes #255