Partial support for AMQP heartbeats

Provide support for AMQP heartbeats in wait_frame_inner. Heartbeats will
be sent/recv'd when this function is called and it enters a
long-blocking wait.

From an library-user's perspective heartbeats are only handled inside of
amqp_simple_wait_frame() (and any RPC function). They are not handled in
amqp_basic_publish() nor anything outside the library

Fixes #86
FIX: NS_PER_SEC should be defined in amqp_timer.h
FIX: struct timespec has tv_nsec member
FIX: add amqp_timer.c to autotools build
FIX: Check to see if linking to librt is required