Measuring time for performance tweaking.

This changes the profiling mechanism and adds some debug logging.

The sys log will then show something like this:
```
[DEBUG] Execution times summary (keep in mind thread_count was set to 4):
[DEBUG]       1 times with average  0.01s = sum  0.01s : ConfigRpmMaker._upload_rpms
[DEBUG]       3 times with average  2.37s = sum   7.1s : HostRpmBuilder._build_rpm_using_rpmbuild
[DEBUG]       3 times with average  0.06s = sum  0.18s : HostRpmBuilder._copy_files_for_config_viewer
[DEBUG]       3 times with average  0.06s = sum  0.17s : HostRpmBuilder._filter_tokens_in_rpm_sources
[DEBUG]       3 times with average  1.84s = sum  5.51s : HostRpmBuilder._save_network_variables
[DEBUG]       3 times with average  0.02s = sum  0.06s : HostRpmBuilder._tar_sources
[DEBUG]      15 times with average  0.01s = sum  0.09s : SvnService.export
[DEBUG]       1 times with average  0.02s = sum  0.02s : SvnService.get_change_set
[DEBUG]       1 times with average  0.01s = sum  0.01s : SvnService.get_hosts
[DEBUG]       8 times with average  0.01s = sum  0.03s : SvnService.log
[ INFO] Elapsed time: 5.58s
```