Adding writeCopy() to Realm

The method `writeCopy()` writes the entire Realm to a new write. During the write, the data is compacted, and the resulting file is smaller. The method is useful when an app migrates to a new version, and it wishes to reduce the file size to a minimum.

@bmunkholm @cmelchior @emanuelez 
You are referencing the files below with file1 file2 file3 anyway. Code would be clearer and take up the same number of lines if you just did:

    Realm.deleteRealmFile(getContext(), "file1")
    Realm.deleteRealmFile(getContext(), "file2")
    Realm.deleteRealmFile(getContext(), "file3")