Adding interposition.json, cleaned up tuf_interface.c

tuf.interposition.json has been added, likely still needs confiuring though.
tuf_interface.c has been updated, the 'configure' function from tuf is now in it's
own C function.  This should offer a nice template for adding additional functionality.

TODO: We need to figure out how to hand information back to Ruby after the C library has
been called.  This is important because functions like 'deconfigure' rely on having a 'configure'
object in order to be called.