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The main result of this change is that libraries are now conditionally loaded
into the packaged app depending on what libraries are used by the components
in the app. (Repl still loads all libraries for all components.)

Adds the new class components/annotations/UsesLibraries.java and revises
scripts/ComponentListGenerator.java and ComponentProcess.java, which are
used to process annotations.

Revised Compiler.java to dynamically construct the classpath and the
dxCommand line from the annotations.

Revised Twitter component to add an annotation:

  @UsesLibraries(libraries = "twitter4j.jar")

Tested on existing projects, such as HelloPurr to confirm that Twitter
library is not loaded into HelloPurr.apk.

10/14: This changes does not affect the Ya version number or the
    Twitter component version.

10/15: Fixed copyright, removed commented-out code, as per reviewer's
    suggestions.

Change-Id: I7acb5d8e2702d47b8f2bdc8f0746ef7d550963c2