Update android target definitions

Review Request #700 — Created July 17, 2014 and submitted — Latest diff uploaded

This is a follow-up based on the comments to merged CR #668.
This can be safely merged without relying on the other two 
open reviews.

This gets rid of a couple of the "sensible defaults". I believe
in them as useful, but they have could proven confusing for anyone
with a non-traditional (or just maven-style) layout.

This requires all android targets to explicitly define a "manifest"
field. Android_resource targets must further define a
"resource_dir". I named it "resource_dir" instead of "resources"
to contrast with what pants expects elsewhere: the tool that
processes android resources expects a directory as input, not
a list of files. The aapt tool spiders down the dir, so the
user must specify which directory holds those resources.

Somewhere down the line, I would like to support common layouts
by default. But the way it was before this commit, it implied
that we supported things that we do not.
local ci.sh passes, android targets can be processed.

Travis looks to be down. [0]

[0] https://travis-ci.org/pantsbuild/pants/builds/30210018