Release v0.7.3, small improvement in yaks-sinatra
Make sure each attribute is only tracked once

When inheriting and adding attributes (include attributes.add(...)),
previously declared attributes would be tracked inside
self.class.attributes twice. This is not a huge problem, but it makes
introspection, and hence pretty-printing, messy.

This commit uniquefies the attribute names.
Fix around hooks, adding second env argument; make Mapper Callable

The pipeline interface has changed to #call(object, env), the lambdas
generated by an around hook weren't accepting the rack env.

Include FP::Callable in Mapper so mapper instances can be treated as procs.