Add *.log to the .gitignore

So I don't check in my adam.log file, accidentally.
Generalizing the Projection class.

Added FieldEnumeration and FieldValue class and trait, to help generalize the
values of schemas in enumerations.

Added tests, to ensure that I can use Projection to project the RDDs of two different schemas
(ADAMRecord and ADAMVariant) using the same class but different field enumerations.