Support move-only attributes

[andreasbuhr]

Boost.Parser avoids copying attributes. So it should be able to handle move-only attribute types.

And this is almost the case, but only almost. A few places in the code prevent it at the moment.

[andreasbuhr]

In #313 I created an example: An expression parser which produces an AST, where the AST nodes are handled through std::unique_ptr. Only a few code places had to be adapted to make it work.

[andreasbuhr]

fixed in #313