Extend the BNC corpus reader with more tags.
BNC code duplicate removal.

.words(), .sents() and their tagged versions use a common method to
obtain concatenated BNCWordViews or result of ._words().

A basic test that covers most of the module is added. An example file
from BNC is taken [1].

[1] http://www.natcorp.ox.ac.uk/docs/eg.xml
[PATCH] Fix handling of comments in _read_xml_fragment (fixes #70)

A fix by Rob Malouf <rmalouf@mail.sdsu.edu>.
Merge branch 'develop' into bnc_test
Merge remote-tracking branch 'origin/develop' into bnc_test
Merge pull request #654 from dimazest/bnc_test

Make BNCCorpusReader to parse the whole BNC