Support RailsAdmin Property class

Refs. https://github.com/sferik/rails_admin/commit/875c9ba7c18665f768bc48f118c0e9d0c74cb81a
Merge pull request #19 from mshibuya/master

Support RailsAdmin Property class
update Contributors
bump to 1.3.0
Fixes syntax issue in README
Merge pull request #20 from benlovell/doc-fix

Fixes syntax issue in README
Merge remote-tracking branch 'upstream/master'