use faster json library if available

[bordingj]

attrs are currently read and written using the standard json library, which is very slow compared to libraries such as ujson (https://github.com/esnme/ultrajson) and pyrapidjson (https://github.com/hhatto/pyrapidjson). I suggest that bcolz should use those if they are available

[FrancescAlted]

Nice suggestion.