Handling "version" field of datapackage.json

[Fak3]

The datapackage.json can optionally contain 'version' field. The datapackage registry will have its own requirements on versioning. At some point we should probably deal with this 'version' field to conform with registry requirements.

For instance, we probably want to make sure that the 'version' in the datapackage.json will match the version of datapackage requested from registry by the user on download.