Epub text extraction and classification

This pull request contains the implemented code for classifying documents published according to the Epub (Electronic Publication) standard.

I apply [minor changes](https://github.com/TellMeFirst/tellmefirst/commit/fccede5c41f6dfd4a30dfbc47e66f1c9ad485e47) to the ClassifyInterface class, in order to simplify the merge of this code in your fork of TellMeFirst.

Beside the usual classification system, we implement a new REST API called */rest/classifyEpubChapters* that provides results of the classification process for each chapter (the top-level section defined in the [Toc](http://www.idpf.org/accessibility/guidelines/content/nav/toc.php)) of an Epub file.

I add the following files for implementing features related to the Epub text extraction and classification:
* [Client.java](https://github.com/TellMeFirst/tellmefirst/blob/d520789a32fe5d96544ffeff835825337c518cfa/back/core/src/main/java/it/polito/tellmefirst/client/Client.java)
* [ClassifyOutput.java](https://github.com/TellMeFirst/tellmefirst/blob/d520789a32fe5d96544ffeff835825337c518cfa/back/core/src/main/java/it/polito/tellmefirst/jaxrs/ClassifyOutput.java)
* [EpubChaptersInterface.java](https://github.com/TellMeFirst/tellmefirst/blob/d520789a32fe5d96544ffeff835825337c518cfa/back/rest/src/main/java/it/polito/tellmefirst/web/rest/interfaces/EpubChaptersInterface.java)
* [ClassifyEpubChapters.java](https://github.com/TellMeFirst/tellmefirst/blob/d520789a32fe5d96544ffeff835825337c518cfa/back/rest/src/main/java/it/polito/tellmefirst/web/rest/services/ClassifyEpubChapters.java)

More details available in the [doc section](https://github.com/TellMeFirst/tellmefirst/tree/ebook-telecom/doc/epub) related to the Epub classifier.