Refactored export

* No disctinction any longer between binary exports and character
exports: we now use the more general "OutputStream" for all cases
* As a consequence, a new DatatablesExport interface has been added. Now
all export classes must implementent this interface. All existing
classes have been updated accordingly
* Remove ExportConfBuilder and added the builder as an inner class of
ExportConf
* The HtmlTable builder has a new method allowing to bind an export
configuration (ExportConf). Only used when setting up export in
controllers.
* Unit tests have been updated accordingly
* Removed area field in the ExportTag and in other objets: it was not
used and won't be necessary since users have now a full control over
what it is exported
* Added ExportUtils.java: it contains utility classes that can be used
when exporting data
* Linked with dandelion/issues#80 and dandelion/issues#131