add change log for version 0.1 -> 0.5
add 0.5.x version to changelog.
Merge pull request #61 from Payum/changelog

Changelog
Merge branch '0.5' into 0.6
0.6 version change log.
Merge pull request #62 from Payum/changelog

0.6 version change log.
Merge pull request #63 from Payum/0.6

0.6