port to aeson 0.7

Aeson 0.7 does not have From/ToJSON instances for ByteString.

This patch encodes keys and secrets to Utf8 and decodes them from Latin1
(There is no encodeLatin1, but fortunately Utf8 is a superset).
whitespace fix
bump version
Merge pull request #5 from ibotty/master

"port to aeson 0.7": This looks great.