As suggested by Peter Dimov on boost-dev ML, scope_check functionality
is merged into scope_exit. Thus scope_exit now has an optional condition
function object, which by default always returns true. If a custom
function object is specified, scope_exit works equivalently to the
previous scope_check, i.e. calls the condition function object to check
whether the action function object needs to be called.
The new scope_check scope guard is a generalization of scope_success and
scope_fail and allows specifying arbitrary conditions for executing the
scope exit action. The scope_success and scope_fail have been
reimplemented in terms of scope_check and now also support arbitrary
fail conditions.
Added exception_checker and error_code_checker conditions for testing
for an exception being thrown and for an error code, respectively.
scope_success and scope_fail use exception_checker by default.
Added tests for the new components.
