vtimer: set custom msg_t.type with vtimer_set_msg

Currently, when using vtimer_set_msg the corresponding msg_t is filled
with the MSG_TIMER ("12345") type.
This approach makes it difficult to differentiate between incoming
messages via vtimer_set_msg.

In this PR I introduce another parameter for the vtimer_set_msg
function to specify a custom msg_t type.
Merge pull request #2162 from cgundogan/vtimer_set_msg_param_type

vtimer: set custom msg_t.type with vtimer_set_msg