combine ElasticPillow with AliasedElasticPillow

nothing else was extending it and it was confusing that they were both
present.
reorder stuff in readme
reorder functions
formatting
add overview docs for how pillows work
use couchdbkit ChangesStream

appears to be faster and more reliable than using requests directly
Merge pull request #93 from dimagi/ChangesStream

use couchdbkit ChangesStream
Merge pull request #92 from dimagi/ptop-cleanup

misc cleanup and documentation