Fix issue with holes in array patterns

Review URL: https://github.com/google/traceur-compiler/pull/348

Closes #348.
Fix issues with array destructuring in arrow and assignment

Fixes #346

Review URL: https://github.com/google/traceur-compiler/pull/349

Closes #349, closes #346.
Fix issue related to the cover initialised name production

Adds support for the cover grammar for `{a = 42}` which is needed in case the object literal is going to be used as a pattern.

We keep track of the cover grammar parse tree, as well as the depth of the assignment expression. When we unwind all the assignment expressions we check if there was a CoverInitialisedName and report an error as needed.

Fixes #350

Review URL: https://github.com/google/traceur-compiler/pull/351

Closes #351, closes #350.
Object.assign and Object.mixin

Added Object.assign and Object.mixin.

Implements [19.1.3.1](http://people.mozilla.org/~jorendorff/es6-draft.html#sec-19.1.3.1) and [19.1.3.15](http://people.mozilla.org/~jorendorff/es6-draft.html#sec-19.1.3.15) respectively.

Review URL: https://github.com/google/traceur-compiler/pull/352

Closes #352.
Support strings as module identifiers

This adds support for strings module identifiers. It still supports the old ident modules but that will be removed eventually.

```js
module './a' { }
```

gets transformed to:

```js
$traceurModules.registerModule('./a', function() { ... }, global)
```

and

```js
import {x} from './a';
```

gets transformed into:

```js
var x = $traceurModules.getModuleInstanceByUrl('./a').x;
```

There is still a lot of work needed. Known issues:

- 'name' paths should be relative to the Project/WebPage
- export module m from './m' is broken. It needs to call getModuleInstanceByUrl outside the getter.
- Need to implement System.get
- Need to implement System.set
- Need to implement Loader normalization and resolve.

Review URL: https://github.com/google/traceur-compiler/pull/344

Closes #344.
Merge branch 'master' into system-get-set

Conflicts:
	Makefile
	bin/traceur.js
	src/codegeneration/ModuleTransformer.js
	src/runtime/get-module.js
	src/runtime/modules.js
	src/semantics/symbols/Project.js
	src/util/url.js
	test/unit/codegeneration/PlaceholderParser.traceur.js
	test/unit/codegeneration/low_level_tests.js
	test/unit/runtime/modules.js
	test/unit/semantics/VariableBinder.js
	test/unit/syntax/ParseTreeValidator.js
	test/unit/util/ErrorReporter.js
	test/unit/util/url.js