Add more to gitignore.

Update to Spree i18n namespace.

Also add missing translation keys, these do not exists in spree_i18n
english version.

Add Swedish locale.

Small improvments to gemspec.

Also apply the Spree 2.0 version style that actual do not require
additional current spree-2-0-stable branch.

Standard additions to Travis config.

Updated readme with build status and contribution info etc.

[Fixes #86]