Removed Boost.ScopeExit from the table on the front page. Comparison with
Boost.ScopeExit is in a separate section, so this table duplicated it
to some degree and detracted focus from introducing Boost.Scope.
Added an example with unique_resource to the front page.
Other wording and example improvements.
Apparently, the Library Fundamentals TS does permit pointers to functions
in scope guards (via the "function object type" definition in the standard),
so instead of taking the effort to preserve the function references decay
those to pointers to functions.
Updated docs and tests.
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.
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.
