Shrink UMD Bundle Size

[brycefisher]

The current UMD bundle weighs in at 346kb. Here's the two biggest dependencies contributing to this size according to webpack:

• buffer 48.6 kB
• evaporate 75.1 kB

Removing, shrinking, or conditionally shimming buffer is the most likely way to improve the bundle size. We'd need to see exactly how it's used by the browserify-crypto package's createHmac() to determine what options are viable.