Support for merge and user-defined contents

• Support for user-defined contents (with custom merge operators)
• Support for merge operations
• Rename IrminTree to IrminNode to reflect the fact that we
  can support arbitrary immutable graphs (it's better if they are
  DAGs but that's not mandatory)
• Rename IrminBlob to IrminContents to reflect the fact that
  we also support structured contents (as JSON objects)
• Support for linking the library without linking to camlp4 as well (#23)