Changing psr/log dependency to 1.0.*

Hopefully this resolves the issue when trying to install this library with
composer: requires psr/log dev-master -> no matching package found

Signed-off-by: Nate Brunette <n@tebru.net>
Updating readme install instructions

Updated to use latest version

Signed-off-by: Nate Brunette <n@tebru.net>
Added additional failure conditions

Instead of only failing on exceptions, we can now fail on different user
defined return values.  There is a new method on the Attemptor interface to
define these.

Signed-off-by: Nate Brunette <n@tebru.net>