Add database and jndi configuration stores.

These will be integrated under the ApplicationConfiguration class in a future revision to pull configuration elements from different sources.
Integrate configuration stores into main Application Configuration component.

Add tests to check for empty config values.
Cache empty values instead of refetching them (until the store is reset).
Add JNDI settings to the functional-tests module.
Code Review

Remove the public ConfigurationStore interface and methods.
Update functional tests' standalone.xml file.
Merge pull request #12 from zanata/jndi-settings

Use JNDI bindings to configure Zanata instead of zanata.properties.