Add support for linked fields

[cmelchior]

Closes #4

This PR adds support for nested members. Getting the syntax right for this is a bit tricky as you want to be able to reference both the top-level field as well as all nested members without name clashing. Some of the things I tried

1) Use underscore for everything
PersonFields.FAVORITE_DOG_NAME;

2) Use special named inner class. 
PersonFields.FAVORITE_DOG
PersonFields.FavoriteDogFields.NAME

3) End fieldname with $ if it is a link. 
PersonFields.FAVORITE_DOG$
PersonFields.FAVORITE_DOG.NAME

4) With 4 there is a slight chance of name clashing if people already use $ for something. 
     It is a bit clearer if the $ is after the separator. 
PersonFields.FAVORITE_DOG.$; // for the "favoriteDog" field in Person.
PersonFields.FAVORITE_DOG.NAME; // All linked fields

In the end I ended on using $ as a "type terminator". This made it possible to use dot syntax to find a nested name. From my experience it is more common to actually reference a nested field than the field name itself, so this seems acceptable.

Feedback is welcome. I made the branch available as 1.1.0-SNAPSHOT so it is easy to test.

repositories {
    maven {
        url 'http://oss.jfrog.org/artifactory/oss-snapshot-local'
    }
}

compile 'dk.ilios:realmfieldnameshelper:1.1.0-SNAPSHOT'