Finish implementing the annotation-building process for WebAnno + add a lot of unit and regression tests

A little bit of gardening: splitting a module growing a little bit too bushy, regrouping some conceptually linked operations

Generalize Annotations to accomodate arbitrary tags (and not only <s/>, <w/> and <pc/>) on arbitrarily nested structure

Draft a very simple WebAnno reader ignoring the fields described in header and taking a lot from TSV and CoNLL

Support CoNLL-X's representation of contractions (labeled with a range instead of a regular id and expanded on the following lines)