Add srtp_set_user_data() and srtp_get_user_data() functions.

Two new functions:

* `void srtp_set_user_data(srtp_t ctx, void *data)`
* `void* srtp_get_user_data(srtp_t ctx)`

It allows the user to associate custom data within a given `srtp_t` context. Useful for later retrieval when a SRTP event is fired (so the user can retrieve the custom data by calling `srtp_get_user_data()` with the `session` field of the `srtp_event_data_t` struct as argument).

This pull request fixes #65.

The commit also includes some minor fixes in the srtp.h documentation (some functions get a `srtp_t` as argument rather than a pointer to a `srtp_t` and vice-versa).