Remove uses of OverloadedStrings

Fixes #6

Though the bytestring docs claim otherwise, version 0.9.2.1 (which ships with
ghc-7.4.2), doesn't include an IsString insatance, which causes build failures
when using OverloadedStrings with ByteString.  This patch removes the reliance
on that feature, restoring compatibility with 7.4.2.