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