Provide hashCode, equals for Composites

[LannyRipple]

Composites, even though a product of value types, do not themselves have value behavior. Provide hashCode() and equals().

Motivating example:

1. Create column family with composite column-names.
2. Create template for column family.
3. Query a slice on the family returning a ColumnFamilyResult.

At this point you cannot specify a new Composite to getColumn() because the implementation uses a Hash and the hashCodes are different for what's in the Hash and what you would hand to getColumn().

[vaidhy]

The problem is that Composites do not know which serializer to use, when de-serializing the data. Check my pull request #503 for one way to handle this.

[LannyRipple]

Since you are having to hand in a constructed Composite to getColumn() it should know how to create a ByteBuffer of its components. The hashCode could be based on the bytes in the buffer. As a mutable structure though that's gonna turn around and bite pretty quick. I'll have to think on this and see if I can come up with what I'm after.