add "alpha" and "type" options to bfast() and its docs


git-svn-id: svn://svn.r-forge.r-project.org/svnroot/bfast@457 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
align with bfastmonitor, use "level" instead of "alpha"


git-svn-id: svn://svn.r-forge.r-project.org/svnroot/bfast@458 edb9625f-4e0d-4859-8d74-9fd3b1da38cb