Plugin infrastructure in corpus2 readers: plugin loading code in pwrnlp/plugin, chanegs all around to accomodate

Revert "add has_more to readers". Not needed with the upgraded shared_ptr swig wrappers (and also not fully correct).

This reverts commit 99388c53.

* make the factory always return writers via shared_ptr
* add a PathWriter wrapper for opening a stream for the lifetime of a normal Writer
* wrap writer factory exceptions
* hide some writer factory implementation details in dteail:: sub namespace
* rename TokenWriter::create to create_stream_writer, add create_path_writer
* bump library version

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

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.