Add IsString instances

[fizruk]

Risking to confuse typechecker even further, we can make some nice IsString instances.
E.g. compare:

at 404 ?~ Inline (mempty & description .~ "Not found")

vs

at 404 ?~ "Not found"

The following types I think can reasonably have IsString instances:

• Host;
• License;
• Tag;
• Param (not sure yet, but "param" would be equivallent to mempty & name .~ "param");
• Response;
• Referenced a (when a has IsString instance);
• Reference (or maybe this newtype can be removed).

@jkarni, @dmjio: thoughts?