Permissible characters in field keys

[e-t-k]

All of the keys we've been discussing so far in #7 and #4 consist entirely of lowercase letters and underscore.
Shall we make this the standard going forward? (Or perhaps we did already and I missed it.)

Possibilities:

• Allow only "word" characters (A-Z, a-z, 0-9, and _ )
• Allow only lowercase a-z and underscore; but allow AZaz09_ in unharmonized keys.

Since we are currently using period as a hierarchy delimiter, the federation members are already going to have to do a cleanup pass on unharmonized keys to remove periods, so further limiting allowed characters seems reasonable.

[jakenoble]

In line with issues #7 and #4, St. Jude and the ongoing API development both use lowercase and underscores for field keys. Standardising this makes sense.

[claymcleod]

Addressed in #19.