Commit 5cf41025 introduced a bug which
prevented client code from being able to clear the date format in
settings. By implicitly wrapping the argument in a `Just` the actual
timestamp rendering kicked in which produced an empty field in the log
output.

This commit fixes this behaviour by changing `setFormat` to take a
`Maybe DateFormat` parameter. Client code can pass in `Nothing` to
disable timestamps.

As discussed in #3, this exposes the `DateFormat` constructor to allow
clients complete control over turning `UnixTime` values into
`ByteStrings`.

Needs GHC >= 7.6 due to System.Environment.lookupEnv

`~~` is an alias of `.` with lower precedence to allow
mixing of `.=` and `~~` without requiring parentheses.

Also remove `logM` et al. and add haddock documentation.

This change makes the buffer size configurable via default settings.
The environment variable "LOG_BUFFER" takes precedence, if set.

Instead of logging only `ByteString`s which means clients have to
perform the conversion from their types, we provide a type-class
`ToBytes` with several common instances.

Also revive 'MonadCatch' instance of 'LoggerT'.