Updates to have scrypt use PHP memory limits memory

Giving this one a try on Travis.

Since memory allocation was moved to emalloc, I removed all the max memory checks in params.c that were to get some form of system memory.  Instead I replaced it with grabbing the current PHP max memory and subtracting the currently used memory.   Since BEST case scenario is for 50% of the memory to be used we can see if that is safe or not.
The pickparams test is probably going to fail since it doesn't have my changes in it:
gettype(scrypt_pickparams(1024, 0.75, 1000));

My limits code is working in raw bytes, wheras this seems to be expecting kB?  So either I need to update my code to convert bytes to K, or these numbers need to be made either much much larger or set to 0.