Issue-144 Allow custom email content

Configuration parameter for custom email template.
Loading of alternate template path.
De-static'ized the filename and content properties of the
VelocityEmailHelper to reflect their dependence on a SeyrenConfig only
available on instantiation.  Further de-static'ized the private method
that performed the one-time content initialization.
Added test locking down default.
Added stub test template for loading verification.
Added test that exercises alternatively configured test template.
Updated documentation.