Memory and execution profiler

Add code for execution and memory profiler. This code is enabled only with Python 3.4+. Two configuration options debug_memory and debug_profile have been added.

Tuned also the tracing messages that are printed. As I have more and more album.ini files with metadata, every scanner run dumps verbose logs but without enough details in case of crash. Now, if it fails while processing a JSON file, I'll know which one has the problem.

The execution report for each album is only printed if tracing level >= 4, and the final one if level >= 3.