Expose certificate verification status to node.js

[bu5hm4nn]

When the DTLS connection is negotiated, mbedtls will attempt to verify the certificate. Depending on the setting, the connection will be refused when the certificate fails or a warning will be issued and the connection established anyways.

To support these two scenarios we need to

1. Expose the verification setting to node js. Specifying this setting should be made mandatory in the API. ( MBEDTLS_SSL_VERIFY_NONE, MBEDTLS_SSL_VERIFY_OPTIONAL and MBEDTLS_SSL_VERIFY_REQUIRED)

2. Expose an optional callback that can be registered from nodejs for the server to be notified about whether verification has succeeded when set to MBEDTLS_SSL_VERIFY_OPTIONAL.

To verify that the setting works, create test/example programs and associated Keys and Certificates that will demonstrate

1. Handshake with only keys and MBEDTLS_SSL_VERIFY_NONE.

2. With MBEDTLS_SSL_VERIFY_OPTIONAL
   a. Handshake with successful verification reported by the callback.
   b. Handshake with failed verification reported by the callback.

3. With MBEDTLS_SSL_VERIFY_REQUIRED
   a. Successful verification
   b. failed verification