add pull and addToSet operators
simplify selectFirst
remove doubtful comment
remove DatatypeContexts usage. closes #302
version bumps
Update Persist.hs

Corrected typo from toJSonText to toJsonText
Update lower bound of postgresql-simple

fromPGArray is only available in postgresql-simple-0.4.0 or newer.
version bump
mongoDB: $pullAll update
add ToBackendKey typeclass

This is needed to implement generic conversion functions
such as toSqlKey for SQL and keyToOid for MongoDB
So the IsSqlKey typeclass was removed,
but the functionality is still there
more explicit documentation for ToBackendKey
ghc 7.6 wants a language extension

perhaps 7.8 only requires it at the call site
no ToBackendKey instance for a (composite) Primary key
version bumps for ToBackendKey
put PersistEntity constraint on right-hand side

This should reduce the need for the user to add
the PersistEntity constraint.

It was on the left side before
890ee388cada5937c393790f84ff0950d4f4b4c1
SqlBackend is no longer a type synonym

Recent persistent changes caused both esqueleto and yesod-persistent to
fail without the TypeSynonymInstances language pragma turned on. While
it's easy to add a pragma, it's also unnecessary in this case. I've
simply made SqlBackend the primary datatype (as was the original
intention) and made Connection a deprecated type synonym.
mongoDB: more detailed error messages
export Key Read/Show functions
fix deprecation warning
rename readMayKey to readMayMongoKey