Add support for serializing custom parameter types

[zolstein]

Add support for serializing custom types that define a particular SerializeNeo4j method.

• Exposes a Packer interface to the public API that data can be serialized into.
• Adds a check for a method SerializeNeo4j(Packer) on parameter types - if it exists, use it to serialize the value.
• Moves serialization logic for dbtype types to SerializeNeo4j methods.
• Rearranges some serialization logic into new Packer methods so that it can be accessed by the SerializeNeo4j methods - in particular for time.Time.