0.4.0.0
* Added several functions to the `Extractors` module:
* `eitherMessageOrValue` from the `ShowErr` module and some others.
* Both conversion functions from `Extra`.
This means less imports in many cases.
* Some changes were made to the `Err` datatype:
* `ErrExpect` was renamed to `ErrExpectContent`.
* `ErrAttr` was renamed to `ErrExpectAttrib` and used for signalling a missing attribute.
* `ErrAttribValue` was added to cater for erroneous attribute values.
* Some field names have been given more descriptive names.
The conversion/validation function given to `attribAs` does no
longer need to return an `Err` value for errors, but only a string
description of the expected value format.
0.3.0.0
* Added `anyContent`
* Added `choice`