Added support for non-android shadows.

src/main/java/org/robolectric/bytecode/Setup.java

  * Modified shouldInstrument() to quickly short circuit for Android
    classes and then check the current ShadowMap for custom shadows.

src/main/java/org/robolectric/bytecode/ShadowMap.java

  * Added constainsKey() for use by Setup.shouldInstrument().

src/main/java/org/robolectric/bytecode/ShadowWrangler.java

  * Made shadowMap "package private", so Setup can access it.

src/test/java/org/robolectric/bytecode/AndroidTranslatorClassInstrumentedTest.java

  * Renamed testCustomMethodShadowed() to testAndroidMethodShadowed()
    and testCustomMethodNotShadowed() to testCustomMethodShadowed().