Provide monadic wrapper for interfaces

[manpages]

While #44 fixes orphan instances issue, making the library compatible with 0.7.3 (and, from I gathered from CHANGELOG, with 0.7.4 as well), it makes API more clumsy by requiring an additional argument to be passed around every time.

This can be played around by either putting implementations (such as element'HTMLElement) into a ReaderT,or by some other more smart monadic trickery I can't think of right now because I'm too sleepy.

I feel, however, that it's not something we should concern ourselves while working on #44.