Handle enumerations

Modello should be able to generate and handle enumerations.

[model]: Add reading of <enumeration> tags. Be able to create integer and string type enumerations.

Integer style enumerations look like this:

public final static FOO = 1;
public final static BAR = 2;

String style enumerations look like this:

public final static FOO = "foo";
public final static BAR = "bar";

[java]: Enumeration classes like [1] should be generated. The model should only return instances of the generated enumeration class.

[xpp3]: When reading and writing enumeration fields serialize and deserialize the enumeration.

[jpox]: Same as for [xpp3].

[xml-schema]: I'm pretty sure xml-schema can handle something like this.

[xdoc]: Add documentation support for the different enumeration elements.

http://svn.apache.org/viewcvs.cgi/maven/continuum/trunk/continuum-model/src/main/java/org/apache/maven/continuum/project/ContinuumProjectState.java?rev=170005&view=log