Package structure

There has been a revision in the package structure within SimpleDBM.

In the earlier structure, the API and the implementation of a module were designed to be together. Hence, if there was a module named test, the packages would have been org.simpledbm.rss.test for the API, and org.simpledbm.rss.test.impl for the implementation.

The current structure is more favoured towards separating the whole of the API from the implementation. In the new structure, all APIs are under org.simpledbm.rss.api.<module>, and all implementations are under org.simpledbm.rss.impl.<module>. I decided to separate the API from the implementation in this way for two reasons:

1. It would be easier to generate JavaDoc for the API alone.
2. The API could be packaged separately in its own jar file without too much effort.