Matrices serialization

I've just implemented support of serialization for la4j.matrix.Matrix interface.

Of course, I've used java.io.Externalizable interface for fast serialization and deserealization mechanism.

Just look at results:

• java.io.Serializable
• Matrix (2048x2048) serialization: 0,337 s
• Matrix (2048x2048) deserialization: 0,476 s
• java.io.Externalizable
• Matrix (2048x2048) serialization: 0,254 s
• Matrix (2048x2048) deserialization: 0,302 s

Externalizable works faster (30% in write and 60% in read) because:

1. It uses just one reflection call (instead of many in Serializable version)
2. It doesn't chose class loader via JVM call