Prevent route blueprint adding duplicates

Addresses #1035
Rename integration-test blueprint to acceptance-test

Addresses #1032
Merge pull request #1043 from jgwhite/rename-integration-test-blueprint

Rename integration-test blueprint to acceptance-test
Merge pull request #1042 from jgwhite/route-blueprint-patch

Prevent route blueprint adding duplicates
Add blueprint listing in ember help generate

Refactor help API around `printBasicHelp` and `printDetailedHelp`.

`ember help` uses `printBasicHelp`

`ember help <command-name>` prints basic and detailed help.

Implement `printDetailedHelp` in the generate command to list out
available blueprints, grouped by package.
Add changelog entry
Merge pull request #952 from jgwhite/generate-list

ember help generate
Add missing descriptions for `build`, `serve`, and `test` commands.
Merge pull request #1045 from rjackson/add-more-help-descriptions

Add missing descriptions for `build`, `serve`, and `test` commands.
Keep --output-path up to date from build and serve commands.

* Adds ability to specify output-path for `ember server` command.
* Removes usage of `broccoli-export-tree` in favor of maintaining a
  single set of logic in the `Builder` model. (previously we had logic
  in both the `Build` task and `loadBrocfile` utility function to keep
  the output directory in sync)