Adding norm_only parameter to load_word2vec_format

When set to True, we only keep the normalised
vectors in memory to save space. The normalised
vectors will also be computed in place to save
temporarily allocating large amounts of memory.
Merge pull request #164 from moustaki/develop

Adding norm_only parameter to load_word2vec_format
print a few dictionary items in str(), as sanity check
added docs for new init_sims(reset) + Yves to CHANGELOG
import six package for Py2/Py3 compat in a single codebase
now importable in Python 3.3 (though most tests fail in 3.x)
banner copy
improve HashDictionary docs
Py3: get rid of print statements
first part of tutorial works in Python 3

Changed file handling in MmWriter; the file produced by the tutorial is
identical in the old version (Py2) and the new version (Py2 and 3).
various Py3 fixes
replace make_closing with contextlib
more Py3 fixes

Some Py3 tests pass now; many choke on str/unicode differences.
Filter wikipedia articles by their namespace
WikiCorpus now uses Main/Article entries only by default (ns=0)
Fix type error
Merge pull request #170 from josephcc/skip_ns

Filter wikipedia articles by their namespace
add josephcc to contributors
Enabling vocab persistence alongside word2vec dump
more doc examples in utils.lemmatize