Add support for Raspberry PI

Hi @creationix,

It should have been an one line change but the shasums for the arm-pi binaries are not available in SHASUMS.txt so I added a few lines to ignore the checksum matching failure. I posted an issue on [joyent/node#5460](https://github.com/joyent/node/issues/5460) so for the next release the shasum should be available.

Cheers

ref #227