Add NeatlineTime element set

[jeremyboggs]

Add a NeatlineTime element set that can, among other possibilities, allow users to specify a date that NeatlineTime can understand based on the value of the DC:Date field, while still allowing any value in DC:Date.

Potential fields in this set:

• Date - with a start and end point.
• DatePrecision - Take David's solution from Neatline Exhibit for signifying date precision.
• Icon - Could allow user to specify a particular icon for this item on the timeline. Could be a select menu reading the contents of 'neatline-time/images/icons', or something similar.

[jeremyboggs]

Closing in favor of using existing DC element set, and adding option to the DC:Date field to help user create date formatting suitable for NeatlineTime.