IPFIX: replace IPFIX_FIELD_TYPES with class; handle signed
IPFIX: replace IPFIX_FIELD_TYPES with class; handle signed

In IPFIX, template fields can be signed or unsigned, or even be pure
bytes or unicode string. This differentiation was extended in this
commit.

Additionally, the IPFIX_FIELD_TYPES dict mapping from int->str was
replaced by a more verbose version, which also includes the standardized
IANA data types. The class' methods provides access to the fixed data
set. This is then used in the IPFIXDataRecord parser.

Refs #25

cooox/python-netflow-v9-softflowd