Removing the use of slash

An oversight on my behalf.  The FromSlash method is used when writing
out the public file name.  There is one place where the slashes are
required which is setting the output file.  I replaced those instances
with filepath.Join which should do the right thing depending on the OS.
Removing GetSection

Using get is not idiomatic to Go.  Also added a bunch of unit testing
around this method.
Unit test support for RenderThing

Externalize the adding of templates for testing.  Created a
helper function that allows templates to be added outside of
the walking of a filesystem.