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.
The log messages are currently 107 to 115 bytes each. However, the size can vary, mostly depending on the length of the host name and the length of the application version string.

The enums provide the tag values, while keeping the code readable and making it less error prone. 

The code isn't coupled because no other functions call the performance probes. 

The tags need to be static to preserve the relation between the tags and the associated database queries. That is what will allow us to query for the performance of specific transactions, specific functions, and specific message types.