Rewrite methods signatures.
map: if rejected, forward promise state and reason.
chain: if promise a or b rejected, forward first reason.
mapError & chainError: map and chain over reason parameter.
mapError & chainError: tests.
mapError: add Promise/A+ compliance and corresponding test.
Add documentation for mapError and chainError.
Merge http://github.com/mudge/pacta
Merge pull request #5 from rbelouin/master

Some methods to work efficiently with rejected promises.