Modifications to load CUDD library on Mac from MacPorts.

Change the loading directions to permit CFFI to find the CUDD library
using its defaulting strategies.  This allows it to find "libcudd.dylib"
on Mac and "libcudd.so" on Linux.

Add code to find libcudd where MacPorts stashes it.  Should be a no-op
if it's not there.
Shadow PI from CL.

One of the CUDD functions has "Pi" as a parameter.  On SBCL, at least,
this causes a crash during compile, as it clashes with the package lock
on CL:PI.
Ignore fasl files.
Fixed example of MAKE-INSTANCE 'MANAGER.
Ignore SWIG-generated file.
Fixed the issue about PI by modifying the package.
Merge pull request #2 from rpgoldman/master

A number of minor improvements

Thanks to rpgoldman!