Merge pull request #22 from amasad/master

Implement getBindings
Tweaks to README.md and Scope.prototype.scan.
Add Node v0.6 back to the .travis.yml list.

This is a follow-up to c502fe451ab07e6fde9ef568b762fd9a481d5e72, where I
moved 0.6 to the allow_failures section but mistakenly removed it from the
list of engines. Hopefully this fix gets the tests running at all again.
Fix .travis.yml syntax.
Bump NPM version to 0.3.16 in preparation for publishing.
Integrate ast-path into ast-types as lib/path.js.

Closes #24.
Bump NPM version to 0.3.17 in preparation for publishing.
Add a simple script to help with measuring traversal performance.
Fix typo in ImportSpecifier branch of Scope scanning code.
Bump NPM version to 0.3.18 in preparation for publishing.
Remove block-style module declaration

Support module x from "y" syntax in scope tracking
Merge pull request #26 from thomasboyt/support-module-declaration

Update scope tracking module keyword support.
NamedSpecifier wasn't defined to take null for name as intended
Merge pull request #27 from thomasboyt/fix-optional-name-for-import-specifier

NamedSpecifier wasn't defined to take null for name as intended.
Fix ExportDeclaration field types and default values.

The main confusion here was that AssignmentExpression means something
different in the spec from what it means in the AST type hierarchy, where
it actually has to involve an assignment operator like = or +=. In the
spec, an AssignmentExpression is just a slight restriction of the general
Expression type, so that's what we should be using here.

Closes #28.
Merge pull request #29 from benjamn/fix-ExportDeclaration

Fix ExportDeclaration field types and default values.
Bump NPM version to 0.3.19 in preparation for publishing.
Stop using Object.defineProperties in the Path constructor.

This makes the .value, .parentPath, and .name properties no longer
immutable, which is regrettable, but it speeds up test/perf.js by a
whopping 34% (from 380ms to 250ms on my machine).
Use path.__childCache instead of (safer but slower) getChildCache(path).

As excited as I was about the privacy guarantees of getChildCache, the
whole point of the child cache is that it should be fast, and implementing
it as a simple property cuts the time taken by test/perf.js by 60% (from
250ms to 100ms on my machine).
Bump NPM version to 0.3.20 in preparation for publishing.