Allow storing void* data into the strp_t session.

[ibc]

When libsrtp fires a provisioned callback it calls it with a srtp_event_data_t struct as argument. The struct contains some fields, including the srtp_t session the event belongs to.

The problem is that I cannot associate my "application SRTP handler" with such a srtp_t session so when the callback is fired I do not know which of my SRTP handlers the session belongs to.

I suggest adding a void* user_data into the srtp_t struct, and two new functions:

void srtp_set_user_data(srtp_t session, void* data);
void* srtp_get_user_data(srtp_t session);

NOTE: This is similar to what other C libraries do. For example OpenSSL provides some functions for this same purpose:

int SSL_set_ex_data(SSL *ssl, int idx, void *arg);
void *SSL_get_ex_data(const SSL *ssl, int idx);

[ibc]

My pull request in #66 implements this feature.