Extract synchronisation data into SyncFile and SyncData classes

- These value objects allow us to read synchronisation data from yaml files that have both strings and symbols as keys.

- Sync files now use strings as keys when serialising to yaml.

Fixes #149
Fix test for yaml output format in ruby 1.9.3
Merge pull request #154 from Locale/issue_149

Extract synchronisation data into SyncFile and SyncData classes