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.
I read up a bit on [openlog](https://www.gnu.org/software/libc/manual/html_node/openlog.html). It's true that this needs to point to memory that is available until `closelog()`. OK, let's see what we can do. `static` is to avoid as much as possible.

One option is to create a `std::string` in `main` and then pass that through the ctor of this class so that it's available until the end of the program.

Is this string contained in every measurement point? I.e. every message has this `opentxs-notary + version`?