add support of script load command by broadcast

[umegaya]

refs #68
this pull request aims to add support for SCRIPT LOAD command, by following fix.

• define msg type for SCRIPT LOAD and parse correctly (8158d2e)
• able to give 'broadcast' attribute to certain kind of msg type, and give broadcast attribute to SCRIPT command (8158d2e)
• do broadcast correctly (cb50953)

also original python test seems to be broken, add new test by shell script (dfba9f8), you can run new test like following:

cd tests/test_redis_sh
./run ./test_script_load.sh

it is only passed limited test case, and I'm very new to twemproxy, so not enough confidence about correctness. can you review and if it looks good, merge to master? thanks in advance.

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