Add the ability to browserify aws-sdk as a dependency.

Works without any custom code. Note that by default only the
default services will be included in the browserified aws-sdk
build. In order to build other services, you can pass the
`AWS_SERVICES` environment variable to browserify:

    AWS_SERVICES=s3,ec2,dynamodb browserify myscript.js > out.js

The above command includes only Amazon S3, Amazon EC2, and Amazon
DynamoDB in the generated SDK. You can use "all" to include all
services.

See #383
Update & refactor dist-tools; removes cache logic.

Caching logic was incompatible with new build format,
as caching worked by generating service cache data outside of
core module. Services are now built inside of the context of
the core module.

dist-tools was refactored to use the new transform function
to drive building of service code.
Ensure license is printed in browserified output
Merge to pull in fix from fd841cf6e31ddd770c447cd0e112c46a47c96165
Merge pull request #386 from aws/npm-browserifiable

Allow SDK to be be browserified without custom scripts

Closes #383