[boost] 1.31.0 closed for changes without permission

Please make no more changes to the RC_1_31_0 branch without the release manager's permission. The one exception is removal of is_abstract code. We need to be totally stable so that the regression tests can cycle one last time. --Beman PS: For the next release, we need to define written "release discipline". What kind of changes are allowed at various points, such as after branch, after release candidates start, etc.