Version 0.31. Added a preprocess() function including documentation for better handling of missing data for lagged covariates. Moved the adjust() and handleMissings() functions along with preprocess() to a new file preprocess.R. Rewritten the adjust() function.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/btergm@32 edb9625f-4e0d-4859-8d74-9fd3b1da38cb