Add support for more Apache types.

Adds support for the following Apache-specific types:
- nil (supported without a namespace by various XML-RPC libraries)
- i1
- i2
- i8 (supported without a namespace by various XML-RPC libraries)
- biginteger

The types are always namespaced when emitted but some of them (nil, i8)
may be used without their namespace in incoming data.

See also #6.