Fix truncation from concat on fixed length str dtypes
Speedup Variable.concat

On this toy benchmark, these changes speed up execution from 240ms to 153ms:

    xs = [xray.Variable('x', [i]) for i in range(10000)]
    %timeit xray.Variable.concat(xs, 'x')
Merge pull request #219 from shoyer/fix-concat-str-truncate

Fix concat str truncation