Added optional libraries with several other small improvements.

- Layered exception-handling in engine
- Can now add several library paths to command line, e.g.
    mickey -L<path1> -L<path2>
- (unix dlopen) library will also search these paths for files,
  so you can put library files in the local directory and do
    mickey -L. foo.scm
- Added an optional library section, for libraries with non-standard
  dependencies.  Started work on a GnuPG library.
Can now specify several library locations, e.g. mickey -Lfoo -L. bar.scm