Allow users to easily provide custom scales

[oubiwann]

Right now, um:scales performs a lookup in the undertheory scales module. We should let users provide their own sequence of notes (in the form of, e.g., (1 3 |#4| 5 b7), etc.). To do things, we'll need to readjust some things and create some new functions:

• rename midi to pitches
• rename get to create
• add a guard to pitches
  • if scale is an atom, convert
  • if scale is a list, don't do a lookup
• update create to use new pitches

This need has arisen as a result of investigating implementation ideas for #16 -- chords are much more varied than scales and definitions are not consistent, so users will need to provide their own notes/forms even more there, than here with scales.

That being said, the majority of the logic will be shared between the two. So this change should take that info consideration:

• what functionality will be shared?
• move that out of um:scales and into um (or wherever else makes sense)
• adjust um:scales as necessary