Performance Metrics Collection

This pull request adds application specific performance metrics collection and logging for the opentxs-notary. 
Please note that this should only be used on linux, since it relies on the rsyslogd and logrotate system utilities to  report the data and rotate the performance log files respectively. 

To use this, run cmake with the -DPERFORMANCE_METRICS=ON option, then follow the setup instructions in  opentxs/scripts/performance_logger/readme.

This is the first installment of our performance metrics tool suite. Future components will insert the log data into a database and provide charting and reporting tools for querying and reporting the performance metrics. We will also add a synthetic load generator and interfaces to Nagios in the future. 

Currently the log data is stored on the local file system, but in future updates, there will be an option to remote it and log to a remote database.
See "Effective C++", item 21: "Use const whenever possible", by Scott Meyers. There are many good reasons for using const given in the aforementioned design guideline. It is sound advice and we should follow it.

It is good to avoid passing objects by const value, but that is not the same thing as avoiding the use of const. It is a lot cheaper to pass a const object by reference because it avoids unnecessarily copying the object. Instead, only the reference is copied. In addition, for small objects, it is cheaper to pass them by value than by reference.