More DatabaseDAO methods

This pull requests suggests adding (and using) the following methods:

  * `deleteEntities`

Full signature is

    void deleteEntities(List<? extends Object> entities)

  * `getEntitiesByStringFields`

Full signature is

    <T> List<T> getEntitiesByStringFields(Class<T> clazz, HashMap<String, String> fieldsAndValues)

  * `getEntitiesByStringField`

Full signature is

    <T> List<T> getEntitiesByStringField(Class<T> clazz, String field, String value)

Also included is a commit that only removes dangling whitespaces and adds newlines between methods.

Please review.