added Field and FieldElement interfaces to allow algorithms operate on any type of field. This change is related to the discussion on dev list http://markmail.org/message/h2rmq6wo7nwqzx4w
commons/proper/math/trunk/src/java/org/apache/commons/math/Field.java
commons/proper/math/trunk/src/java/org/apache/commons/math/FieldElement.java