encoder: Add benchmarks; Fixes #7

BenchmarkDecodeFloat32	     500	  16320072 ns/op
BenchmarkDecodeFloat64	     500	  19265097 ns/op
BenchmarkDecodeUint8	     500	  14820210 ns/op
BenchmarkDecodeInt16	     500	  15722880 ns/op
BenchmarkDecodeInt24	     100	  52042048 ns/op
BenchmarkDecodeInt32	     500	  15294895 ns/op
BenchmarkDecodeALaw	     500	  18334217 ns/op
BenchmarkDecodeMuLaw	     500	  14521286 ns/op

BenchmarkEncodeFloat32	    1000	   6488006 ns/op
BenchmarkEncodeFloat64	    1000	   9089174 ns/op
BenchmarkEncodeUint8	    1000	   9136655 ns/op
BenchmarkEncodeInt16	    2000	   3424256 ns/op
BenchmarkEncodeInt24	    1000	  10707008 ns/op
BenchmarkEncodeInt32	     500	   9236220 ns/op
BenchmarkEncodeALaw	    1000	   8616411 ns/op
BenchmarkEncodeMuLaw	    1000	   7751985 ns/op