Merge pull request #674 from gonzaloUr/add-hashcode

...dcore/src/main/java/com/xpn/xwiki/objects/BaseObject.java

...c/main/java/com/xpn/xwiki/objects/BaseStringProperty.java

...e/src/main/java/com/xpn/xwiki/objects/NumberProperty.java

...e/src/test/java/com/xpn/xwiki/objects/BaseObjectTest.java

...st/java/com/xpn/xwiki/objects/BaseStringPropertyTest.java

...c/test/java/com/xpn/xwiki/objects/NumberPropertyTest.java