MeshState: add initial implementation; see #21
MeshState: add tests and benchmarks; see #21

BenchmarkMeshState	100000000	        11.4 ns/op
BenchmarkMeshStateEqual	100000000	        16.2 ns/op
BenchmarkMeshStateDiff	100000000	        14.1 ns/op
Mesh: add Append implementation; see #21
Mesh: add Append tests and benchmarks; see #21

BenchmarkMeshAppend100Prealloc	 1000000	      1821 ns/op
BenchmarkMeshAppend100Dumb	  500000	      3458 ns/op
BenchmarkMeshAppend1kPrealloc	 1000000	      2040 ns/op
BenchmarkMeshAppend1kDumb	  200000	      9744 ns/op
BenchmarkMeshAppend4kPrealloc	  200000	     10472 ns/op
BenchmarkMeshAppend4kDumb	   50000	     64772 ns/op
Merge pull request #22 from azul3d/v1.1-meshappend

add MeshState and Mesh.Append