Adding functions pCompose and pPipe.

R.pCompose works like R.compose, but handles promises.
The resulting function will evaluate to a promise if
any one of the composed functions returns a promise.
If all of the composed functions return something other
than a promise, then the behaviour is identical to that
of R.compose(). In this sense, R.pCompose() is backwards
compatible with R.compose.

R.pPipe is like R.pCompose but takes the arguments in the opposite order.
Merge pull request #524 from yuri/master

Changing _compose() to support promises.