Support @steal in slim build

[imaustink]

Potentially unhandled rejection [1] Error: Cannot create slim build. "@steal" module is not supported
    at isAtSteal (/Users/austinkurpuis/bitovi/bitcentive/public/node_modules/steal-tools/lib/slim/checks/steal.js:32:9)
    at Array.forEach (<anonymous>)
    at /Users/austinkurpuis/bitovi/bitcentive/public/node_modules/steal-tools/lib/slim/checks/steal.js:21:21
    at Array.forEach (<anonymous>)
    at module.exports (/Users/austinkurpuis/bitovi/bitcentive/public/node_modules/steal-tools/lib/slim/checks/steal.js:10:14)
    at checkSupport (/Users/austinkurpuis/bitovi/bitcentive/public/node_modules/steal-tools/lib/stream/check_slim_support.js:20:2)
    at DestroyableTransform._transform (/Users/austinkurpuis/bitovi/bitcentive/public/node_modules/steal-tools/lib/stream/check_slim_support.js:10:15)
    at DestroyableTransform.Transform._read (/Users/austinkurpuis/bitovi/bitcentive/public/node_modules/readable-stream/lib/_stream_transform.js:182:10)
    at DestroyableTransform.Transform._write (/Users/austinkurpuis/bitovi/bitcentive/public/node_modules/readable-stream/lib/_stream_transform.js:170:83)
    at doWrite (/Users/austinkurpuis/bitovi/bitcentive/public/node_modules/readable-stream/lib/_stream_writable.js:406:64)

This is preventing Bitcentive from taking advantage of the slim build, as many of its dependencies, for example, steal-socket.io, use @steal.

[imaustink]

cc @matthewp @m-mujica

[matthewp]

Issue moved to stealjs/steal-tools #849 via ZenHub