added value_s, main reads stdin, renaming

everything updated to use value_s instead of num and expr stack
created value_cmp to compare value_s
GArray now contains value_s instead of doubles
added value_s to makefile

main now reads arguments from stdin
changed/removed expr helper functions
expr and value copy functions take const ptrs
push_num snprintf code removed

isDuplicate renamed to is_duplicate
GArray nums renamed to operands
solution renamed to target_num