fix #18
Fixed issue #20 APCu's APCIterator constructor

is not compatable with APC
Fixed iterator tests affected by issue #20
Assert cache clear only when 'user' was passed #18
Fix #18, #20 assert 'user' keyword
missing include
fix type casts time_t vs ulong
Fixed the gc routine.

To avoid negative now - dtime, now should be updated as often as possible. A cache item could be deleted by another thread/process.
fixa apc_clear_cache signature, #18