Added new tests for ParseEvent.
The slugger now trims the original string.

This is a (slightly) change in the previous behaviour of the slugger,
bay makes a lot more sense.
Book configurator now parses the second level options.

I still don't know how to parse recursively a multi-dimensional
array with PHP and apply a callback to each value ... but at least
the code now parses first and second level options (it's very uncommon
to use third or deeper level configuration options).
Tweaked a minor error message.
Improved the code of the publishers and added more tests.

The code of some publishers (specially HtmlChunked and Epub2) was a
bit messy, with very long methods, bad variable naming and few tests.
This is the another step into the overall goal of improving easybook
code quality.