[registry] simplify registering StorageExtensions.
[doc] simplify get it started doc.
[doc] fix links. clear what module for.
fix for cordoval comments
Merge pull request #4 from Payum/doc-simplify-usage

Simplify usage of the module.
fixes after repositories were merged.
[composer] update description
[doc] fix the link to supported payments page
release 0.7
master is 0.8-dev