Add decoder implementation with tests.

[meetps]

I have added the decoder implementation with tests currently supporting

• Branch instructions
• Jump Instructions
• Upper Immediate Instructions
• Arithmetic Immediate Instructions
• Extended Arithmetic Immediate Instructions
• Arithmetic Reg2Reg Instructions
• Extended Arithmetic Reg2Reg Instructions
• Load Instructions
• Store Instructions
• Fence Instructions

The decoder currently supports RV32I Instruction Set.

I am adding support for other advanced 32 and 64 bit instructions currently.

[jck]

I've merged this for now. A few notes:

• Please use a branch instead of master from now on and keep your master synced with mine: https://help.github.com/articles/syncing-a-fork/
• Any particular reason why you used nosetests instead of py.test?
• rewrite your tests to use simple py.test style asserts instead of unittest's assert methods.