renaming OBJECT_MERGE_UNDEFINED -> OBJECT_MERGE_UNDEFINED_VALUE

small doc fix

adding a few more test cases and enabling "NULL" to be considered as …

…"UNDEFINED" when merging

adding context and depth tracking for exception messages, and cleanin…

…g up internal api

using SORT_REGULAR flag for optioned array_unique call, and adding ne…

…sted array value merging, and fixing bug when types differ in recursive merge

fixing functions and removing callback work; will require additional …

…thinking

efficiency improvments and using "property_exists" for existence dete…

…rmination over "isset" on objects

some more complex nested tests

improved recursion and updated api

fixing bug in mergeObjectValues