Teach the MAGMA interface how to handle GF(q) conversions

[malb]

The attached patch implements conversion to/from MAGMA polynomials over non-prime finite fields.

[malb]

Forgot to mention:

• please review carefully, this includes a change to SageObject? and FiniteField?
• 'make test' passes (didn't try optional because I don't have all optional packages installed)

[was]

The patch adds a couple of debug print statements, e.g.,

  +        print "INPUT",x

[malb]

Replying to was:

The patch adds a couple of debug print statements, e.g.,

  +        print "INPUT",x

and removes them again in the second patch in the file. sage -upgrade committed my unfinished business.