provide `with-cause` to allow specifying the cause explicitly

previously the cause was automatically captured within a try+ catch
clause or nil otherwise.

This could also be accomplished by allowing more flexibility in the
throw+ arguments, but that's quite a bit more complicated to code.
abandon with-cause in favor of optional arg to throw+
update throw+ docs
update README
fix doc string typo
Merge pull request #48 from scgilardi/allow-specifying-the-cause

provide optional argument to throw+ to allow specifying the cause explicitly, addresses issue #29