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.
The new row of examples illustrate explicit deactivation of the scope
guards, as opposed to automatic failure detection via scope_fail.
Also improved the other examples to look more realistic.
Closes https://github.com/Lastique/scope/issues/13.
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.
