Peter M. Groen / openbr

This allows e.g. writing .mat files as directly as output of compare, as well
as writing similarity matrices as images (although this could use some scaling)

Signed-off-by: caotto <ottochar@gmail.com>

Export the facevacs directory found by cmake, use that to define a default
config file location of facevacs_dir/etc/frsdk.cfg. Store the config file
path as a property of Globals, allowing users to specify a config file path
as a command line argument to br.exe.
Modifications to openbr_plugin.cpp allow adding dynamic properties (strings
only) to br::Objects.