spec 1.4: `component.version` optional

[jkowalleck]

from spec 1.4 on the Component's version is no longer required.
therefore it may be undefined and can be omitted when normalizing to JSON or to XML.
an empty version string is still valid.

feature:
make version optional in setters.
make version default to undefined

[vanthome]

when will spec 1.4 be supported? It's already supported by the npm flavour of this module as it seems.

[jkowalleck]

there is no CDX 1.4 support in this package. nobody implemented it, yet.

it has not been decided how the future of this package will look like.
i suggested to the board a plan as described here: #321