modularize async utils

[koulmomo]

@akshayp @ZeikJT

As discussed, this refactor modifies the API and moves all methods that used to be accessible under the root package, to require('fluxible-action-utils').async as well as allow for modularized requires.

#8 should be refactored to follow the new exports/directory structure presented here, and only merged after this PR. Will then release both as part of a 0.2.0 release.

New modularized API

var utils = require('fluxible-action-utils');  // returns { async: ... };
var asyncUtils = require('fluxible-action-utils/async'); // returns {executeCritical: ..., executeMultiple: ..., ...}

ustils.async === asyncUtils // true!

var executeMultiple = require('fluxible-action-utils/async/executeMultiple');
(utils.async.executeMultiple === asyncUtils.executeMultiple) && 
(utils.async.executeMultiple === executeMultiple) // true!

// ...

🎨 refactor codebase to support modularized API and the addition of more utils.
✅ add new unit test(s) for modularized exports + getTasksForAsync method (which was broken out to /internals/getTasksForAsync).

[yahoocla]

CLA is valid!

[coveralls]

Coverage remained the same at 100.0% when pulling 3f601d9 on refactor/modularize-async-methods into 3d2f6f0 on master.

[koulmomo]

Anything under the internals directory should not be explicitly exported and should not be considered stable (ie, require externally at your own risk as breaking changes inside internals will not be considered breaking for the lib).

[coveralls]

Coverage remained the same at 100.0% when pulling bd27d82 on refactor/modularize-async-methods into 3d2f6f0 on master.

[coveralls]

Coverage remained the same at 100.0% when pulling bd27d82 on refactor/modularize-async-methods into 3d2f6f0 on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 271ab55 on refactor/modularize-async-methods into 3d2f6f0 on master.

[akshayp]

This looks good to me @koulmomo 👍 💯

[akshayp]

Also I don't see why this can't lead to a 1.0 bump