Reader factory. There is some code duplication with the writer factory as well as among the individual Readers, but overall it is now easier to support more input formats.

initial switch to use shared_ptrs for sentence and chunk objects, ale get rid of the pointless sentence template

In case the xces document contains sentences with no chunk containing them, a fake chunk will be created to hold the sentences.
Previously this situation threw an exception.

Libcorpus2 now contains the base strructures (tokens etc), their I/O and related classes.
Libpwrutils (name TBD) contains basic building blocks common to all tagger-related libraries
Tagset-tool only needs libcorpus, so was brough here as well.