Enabling vocab persistence alongside word2vec dump
Parameterize LdaModel variational magic numbers

This commit removes the VAR_MAXITER and VAR_THRESH constants from
LdaModel and allows each to be set when a new model is made, or
during a call to update(). They have been named iterations and
gamma_threshold.

Logging messages for the number of passes were changed to use the word
"pass" instead of "iteration", to avoid any confusion. Other logging
messages were also updated to include these vars.

This is a non-automatic fix for Issue #58.
add Christopher to CHANGELOG authors
remove relative imports, fixes #167
use smart_open for word2vec files
add Yves Raimond to CHANGELOG