Implemented a MemoryMonitor to debug memory consumption issues
Merge branch 'master' of github.com:gem/oq-engine into performance-monitor
Enhanced the PerformanceMonitor and relative tests
Added forgotten file
Merged from master
Added python-psutil dependency
Merged from master
performance:

Recover gracefully if psutil can't read process memory info due to an
AccessDenied error. If this happens, the process in question is discarded
from the monitored and is not checked anymore.
Various fixes according to Luigi suggestions
Small improvements to the PerformanceMonitor
Removed a debug statement
Merge pull request #1054 from gem/performance-monitor

Performance monitor to measure times and memory occupation of bottleneck code