Add "beststep.r" by Patrice

Also modify "DESCRIPTION" by adding "bestglm" to "Depends".
Modify "beststep.R" and add man-page

Add argument "topm" to function "beststep".
Add man-pages for function "beststep" and "selectmode".
Change "library" to "require" in "beststep.R"
Merge branch 'beststep'