Merge pull request #27 from clintongormley/master

Updated docs to reflect new functionality
0.08 release stuff
Merge branch 'master' of github.com:stevan/promises-perl
release engineering for v0.90
adding in some details to make Dist::Zilla happy
fixing some overload bugs
A single pre-resolved promise passed to collect fails
because $all_done is already resolved.

Fixes #28