JSON serializer/deserializer for docs with full fidelity

[klehmann]

• purpose of this serializer is to have a replacement for binary DXL
• uses BASE64 encoded item values by default
• might be extended later on for values that are easy to convert into JSON with full fidelity, e.g. dates, numbers and strings without special characters or newlines
• newlines in string item values need special treatment, because depending on the context they are replaced with \0 (normal docs) or kept (e.g. LS code, MIME items)