change branching model

Change branching model to follow the model described here
http://nvie.com/posts/a-successful-git-branching-model/
update readme

add link to semantic versioning 2.0.0 http://semver.org
Merge branch 'hotfix-1.2.2' into develop
Bumped version number to 1.2.3
fix for #3 Undefined variable: text_sizes in product page
Wrong url return from admin product controller (adamjm7)

When filtering by printable status, the url returned was incorrect.

Multiple lines contain
'$filter_printable=' . $this->request->get['filter_printable'];

It should be
'&filter_printable=' . $this->request->get['filter_printable'];
Merge branch 'hotfix-1.2.3'
Merge branch 'master' of https://github.com/opentshirts/opentshirts