Need to be aware of Java class file versions when writing data

[kylestev]

Instructions and constant pool entries/types that are not implemented in the JVM version for a class file should not be able to be written to a file.

At parse time, this library does not modify entries in the constant pool nor does it optimize instructions for later versions of the JVM spec. There needs to be an error thrown when adding instructions to a class method or a constant pool entry that does not conform to the current class file's JVM version.

[kylestev]

by merging #14 we only need to worry about verifying the constant pool entries as well as the attributes