Introduce a logging function into aim_log_t,
so that aim_log_t does not rely on aim_pvs_t internally,

Rework aim_pvs_* to provide logging functions for use with aim_log_t,
if so desired.
Consolidate pvs log functions.
Specify description when setting log function.
Remove logf field from aim_pvs_t.
Return NULL from aim_log_pvs_get if the pvs is not aim_pvs_logf.
Merge into master from pull request #30:
AIM logging function (https://github.com/floodlight/infra/pull/30)