Fix return type assertions & arrow functions

Fixes #1443
Allow undefined/null as source for Object.assign

Fixes #1448
Add parsing support for type generics

Aka TypeArguments. The syntax is taken from the TypeScript spec
and here are some examples:

Array<number>
Map<number, boolean>
a.b<c, d.e, f.g.h>
Correct the relative path for traceur-loaded nodejs modules.

Using Module._compile() causes evaled code to require() relative to the
caller of _compile(). We want the require() to be relative to the file containing
the es6 code we transcoded.

Here we include a file-local function `require()` during transcoding of modules.
The local require() calls a runtime function with the module name and the required path.
The runtime function resolves the required path iff its relative.

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/1434

Closes #1434.
Add missing file from previous commit.

TBR=@arv

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/1454

Closes #1454.
Make types work in placeholder parser

This requires the parser to take an option object.
PlaceholderParser: Allow null in type context

This is so that you can do:

  parseStatement `var x: ${null};`

result in

  var x;
Add support for class member variable
Type asserts: Fix issues with TypeName and asserts

The assert functions we used for testing was wrong. It did not
handle number vs Number.

We also dropped the `moduleName` of the TypeName on the floor.
Add test case to verify compile to register format with name

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/1470

Closes #1470.
Update README.md

Fixed link to wiki Getting Started page
0.0.73
Type asserts: Add support for symbol primitive
Generic type assertions

This adds basic support for generics for the type assertions pass.
Right now the responsibility for validating the type is part of
the test. All Traceur does is to transform

T<x, y>

into

$traceurRuntime.genericType(T, x, y)

which now constructs a new $traceurRuntime.GenericType which the
testing asserts can inspect to determine how to validate the value.
Support sourcemaps in node execution.

Add dev-dependency on source-map-support.
Add --sourcemaps=memory option to store sourcemap in memory (actually
to not store it as data URL or file on disk.
When sourcemap option selected, supply sourcemap out of memory cache.
Replace Loader.getSourceMapInfo() based on codeUnit lookup with
Loader.getSourceMap(outputName).

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/1475

Closes #1475.
Remove unused code from PR #1475

TBR=@arv

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/1479

Closes #1479.
Allow >> in type generics syntax

We support this in a similar way to the way parsing regexps work.
When we look for the `>` we let the scanner treat `>>` as `>` and
decrease the position of the scanner so that next token it sees
starts after the first `>`.

Fixes #1467
Allow .js at the end of module names

Issue #1221
0.0.74
Use .js for module names

Issue #1221