Add syntax and semantics for metadata

Many systems have metadata such foreign keys, primary keys, sortedness, and uniqueness.

kdb+ has an additional attribute called "grouped" which indicates a large amount of repetition.

http://code.kx.com/wiki/JB:QforMortals2/tables#Attributes

Questions:
- What metadata do we want to support?
- What syntax do we want?
- How general do we want attributes to be?
- Should `ndarray`s support sorted and unique?
- I think that only collections should have these attributes
- Should we be able to set these attributes later, i.e., after a computation has occurred, or just when we're constructing the datashape?

I'll be posting what other systems do regarding these, and any interesting tidbits that turn up.

Personally, I like the name "attributes", so that's what I call them when discussing them here.