Changes for proof-of-concept AS req building

Change imports so we don't need to keep saying univ. or char. or
useful.  Add kludgy workaround for pyasn1 limitation on assigning
pyasn1 objects to tagged sequence fields.  Rename "type" to "asn1type"
in _mfield and _ofield to avoid shadowing the built-in function
type().  Add principal name-type constants; these might move to a
different subpackage later.
Add .gitignore