Added Bytecode Manipulation section with some content.

The ASM, cglib, and Javassist are the three most commonly used libraries for working
with Java bytecode at runtime. They are used by many frameworks and programs to
provide dynamic behavior like AOP programming.
Merge pull request #45 from acisternino/bytecode-manip

Added Bytecode Manipulation section with some content. Maybe remove cglib later on, I think it suffers from inactivity.