Field: allow multiple data fields per asset

[mice-sk]

Use case/examples:

• image assets may need URIs for multiple image resolutions (full-size pic, thumbnail, etc - for environments where Assetic is not in use)
• video assets may need an additional parameters for storing still image URI

[mice-sk]

Note: I removed mention of different video providers from the second bullet, as this is now handled by different field types (e.g. TYPE_ASSET_VIDEO_FLV, TYPE_ASSET_VIDEO_YOUTUBE).