Create ByteValueVertex to store vertex values as bytes without object instance

Facebook task (bootcamped) #4320492

This modification creates an alternative to DefaultVertex that stores the vertex value as raw bytes and only realizes the object when getValue() is called. The purpose of this change is to reduce memory consumption when vertex values are large.

User must define their own BytesToValueConverter class to translate the object type to bytes and back. I created ones for float and doubles. Using Java's serialization is bad idea as it adds a lot of overhead (for example full class names). However, one could easily create a generic bytevalueconverter that uses java serialization.

Configuration -vcl is used to define the vertex class and -bc the converter. For example:
-vcl org.apache.giraph.graph.ByteValueVertex -bc org.apache.giraph.graph.byteconverters.DoubleConverter