Implement custom bucket mapping for scripts.

This commit introduces the new customBucketMap settings that is a Map,
mapping a String (the name of the bucket to push to) to a List of
Strings (the files that should go to that bucket instead of the default
bucket specified in the awsS3Bucket setting).
Abstract out bucket mapping.

DRY, ya hurd?
Don't use customBucketMap as a setting.

We're going to be specifying this routing in the bulde file itself,
instead.
Implement bucket mapping for CSS deployment.
Match yo braces.
Implement population of the customBucketMap from the bundle files.

Bundle files are typically of the following format

  bundlename
  filename1.js
  filename2.js

With this commit, we change that format to allow the specification of a
custom destination bucket, like so:

  bundlename->mycustombucket
  filename1.js
  filename2.js

During deployment, this bundle will be sent to that bucket instead of
the one specified in build.sbt.
Properly include a dot before the extension in bucket mapping.
Correctly filter bundles for default bucket.
Let's not deploy everything to the default bucket twice.
Mention bucket targeting in README.
Refactor bundle target parsing based on code review.
Style tweaks based on code review.
Use a partial function in place of _'s. (Code review).
Remove the toMaps, as per code review.
Merge pull request #11 from farmdawgnation/grand-central-station

Ability to deploy individual bundles to different buckets.

Implements the ability for someone to specify a different destination
bucket for particular bundles. Bundles who have been targeted at a
specific destination bucket will be excluded from the deployment to
the bucket specified in the awsS3Bucket setting in build.sbt.

To target a bundle at a particular bucket, you use an arrow designation
in the bundle file. Example:

  primary-scripts
  script1.js
  script2.js
  script3.js

  aux-scripts->aux.frmr.me
  aux1.js
  aux2.js