Refactoring of custom SSL context

[stklcode]

The connector supports the declaration of custom trusted CA certificate for the HTTPS endpoint since v0.4.0 (#2). This has been implemented storing a whole SSLContext in the connector instance.

This has several side effects, if only the CA cert is of interest, so I decided to refactor this.

Now the connector only stores the X509Certificate object and creates a SSLSocketFactory as required.

(this issue is just opened for documentation/discussion purposes, as the implementation is already done)