Add an `assert-valid` option to daisy3-to-epub3

This option controls whether strict RelaxNG validation is applied to the
input DTBook. The default is `false`.

Fixes [issue 273](https://code.google.com/p/daisy-pipeline/issues/detail?id=273)
Merge pull request #24 from daisy-consortium/issue-273

Add an `assert-valid` option to daisy3-to-epub3