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

add two new writers: a dummy "none" writer that disables output and a "stats" writer that outputs general info only about token counts and annotation info

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.

Fast and drity (regex-based) XCES parser based on the one in Pantera, noticeably faster but prone to breakage on even slightly freeform xml files