Implement System.semverMap() and test; set package map.

For example, traceur@0.0.13 is aliased by traceur, traceur@0 and traceur@0.0.
Change the intermediate build referrer to a valid semver.

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/718

Closes #718.
Report an error on circular dependencies.

Add a test for circular dependency.
Detect and report useful error message:
"Unsupported circular dependency between feature/Modules/ImportCircular.module
and feature/Modules/resources/clockwise"
See issue #684.

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/728

Closes #728.
Clear the ModuleStore cache when registering modules

Throw on duplicate calls to ModuleStore.registerModule.
Add test for duplicate calls to ModuleStore.registerModule()

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/715

Closes #715.
Remove the hack to allow .js module names.

This hack was to be transitional only.
Update semver

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/730

Closes #730.
lib/system-map.js from https://github.com/guybedford/systemjs.git.

Copied at beae8195d. License MIT.

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/726

Closes #726.