Re-organize the Feature schema

[tdanford]

This is a ticket to capture our collective thinking around the re-organization of the Feature schema. The Feature schema needs to be edited to satisfy the following (additional) requirements:

• it needs an explicit 'type' field
• it should be less file-format specific (i.e. fields like 'qValue' and 'signalValue' could be moved to an attributes field)
• we need to make sure it's as memory efficient as possible (and some benchmarking would be nice, too)

[laserson]

Closing in light of #30