Moved all specific minecart entities to sub-package.

This change is breaking for the new API for 1.5, including the interfaces for
the three new Minecart types and the name of the previously TNT_MINECART
material.

This change also deprecates the two previous specific minecart types located
in the org.bukkit.entity package. This deprecation is not a breaking change
and will still be internally supported.

Each minecart type has new javadoc to be slightly more descriptive. Included
with this are specific references to the interface for each respective
EntityType entry. Another package-info.java file has been included as well.

All specific minecart types extend minecart, each with a more descriptive
name. The naming will also follow the old convention. In addition, the
minecart with no specific designation is now more closely referred to as a
rideable minecart.