Add offset_bottom support, test and small fixes

[balasan]

Adding support for offset_bottom and corresponding test

Also a fix for overscrolling (as in chrome browser)

And a fix for inconsistent behaviour in two.html test *needs to be checked (line 30 in .coffee file)

*please ignore the Makefile change

[rimaseturas]

+1

[R4YM3]

+1

[danyaPostfactum]

+1

[vinnysgreen]

+1, did not mean to review.

[iamtekeste]

Is there anyone around to merge this?

[dwoznicka]

@leafo are you still maintaining the library? if not, is it possible for you to find someone who will? we really need this option to be merged, waited a long time :)