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.
This avoids a potential confusion that the object in the examples
is a scope guard, as scope guards have the set_active method.
Closes https://github.com/Lastique/scope/issues/1.
Based on the boost-dev ML discussion, added a note regarding exception_checker
limitation with coroutines. Also, rearranged docs and improved wording in
a few places to make exception_checker and condition functions in general
documentation more distinct. Added rationale for using conditional scope guards
as opposed to using scope_exit universally.
