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.
What do you have against const values?