Add encoder/decoder with simple string based definition

[axic]

I like it (as described in #2), but not sure it should be included.

Sample code:

var tmp

tmp = abi.simpleEncode('someMethod(bytes,uint):(bool)', '12341234234', 1234)
console.log('encoded method', tmp.toString('hex'))

tmp = abi.simpleDecode('someMethod(bytes,uint):(bool)', tmp)
console.log('decoded method', tmp)

Output:

encoded method 2e1b8e21000000000000000000000000000000000000000000000000000000000000004000000000000000000000000000000000000000000000000000000000000004d2000000000000000000000000000000000000000000000000000000000000000b3132333431323334323334000000000000000000000000000000000000000000
decoded method [ false ]

Works with arrays too someMethod(bytes,uint[]):(bool):

encoded method d75ceaf400000000000000000000000000000000000000000000000000000000000000400000000000000000000000000000000000000000000000000000000000000080000000000000000000000000000000000000000000000000000000000000000b3132333431323334323334000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000200000000000000000000000000000000000000000000000000000000000004d20000000000000000000000000000000000000000000000000000000000000001

[coveralls]

Coverage decreased (-4.2%) to 78.808% when pulling 6c80b49 on feature/simple-encoder into 79aaec5 on master.