allow slices to be sent to specific workers resolves #624

[jsnoble]

this needs changes in the queue module ts-archive/queue#5

[kstaken]

Integration tests are failing on this branch. One error was fixed by doing a rebase but I'm still getting this:

Unhandled rejection TypeError: Cannot read property 'workers_joined' of undefined
    at /Users/kstaken/projects/opensource/test/teraslice/integration-tests/spec/wait.js:83:44
    at tryCatcher (/Users/kstaken/projects/opensource/test/teraslice/integration-tests/node_modules/bluebird/js/release/util.js:16:23)
    at Promise._settlePromiseFromHandler (/Users/kstaken/projects/opensource/test/teraslice/integration-tests/node_modules/bluebird/js/release/promise.js:512:31)
    at Promise._settlePromise (/Users/kstaken/projects/opensource/test/teraslice/integration-tests/node_modules/bluebird/js/release/promise.js:569:18)
    at Promise._settlePromise0 (/Users/kstaken/projects/opensource/test/teraslice/integration-tests/node_modules/bluebird/js/release/promise.js:614:10)

[kstaken]

Integration tests require a rebase to master but then pass.