New json crate

[Enet4]

This is a preliminary pull request for what will become the dicom-json crate: a piece to serialize and deserialize DICOM content to/from JSON.

Resolves #128.

This draft already includes:

• Serialization of DICOM-rs data structures such as InMemDicomObject and DefaultDicomObject to a JSON string or a JSON value
• Serialization of values in the Other category of VRs (such as OB, OW, OF, ...) in base64.

Missing:

• [x] Deserialization from JSON
• [ ] Serialize pixel data fragment sequences (non-standard, won't do for now)