Code cleanup.

Summary:
Removed unused code/imports and a missing generic.
Fixed a unit test. Unit test methods should never have return statements.

Test Plan: Eliminated all warnings in Eclipse.
Introduce PMD 5.0.3 to catch source code violations in Buck's source code.

Summary: This was motivated by javac's inability to warn for unused imports.

Test Plan: Sandcastle builds
buck test: Warn when no tests ran

Summary: Warn users if they didn't actually run any tests

Test Plan: that'd be nice
Remove a use of Hungarian notation.

Summary: See http://facebook.github.io/buck/contributing/codestyle.html

Test Plan: Sandcastle builds.
Fix some bugs in RuleKey calculation in android_resource() and java_binary().

Test Plan: sandcastle builds
Try to make it easier to determine the license for Buck (Apache 2.0).

Summary:
Add a LICENSE file downloaded from http://www.apache.org/licenses/LICENSE-2.0.txt
Updated README.md to state the license information explicitly.

Test Plan: Previewed README.md locally using a Markdown Chrome extension.
Clean up some error messages thrown when writing bad BUCK files.

Test Plan:
Unit tests,
Tried repro stesps in https://github.com/facebook/buck/issues/6

Fix lint

Test Plan: ant lint