Add option project.default_exclude_paths

Summary:
This adds the ability for a repo to exclude one or more
paths from processing in 'buck project'.

This is useful for repositories that have sandbox directories
that hold Buck projects which may or may not build at any
given time. (Unlike 'project.ignore', an excluded path can
still be built if specified explicitly).

Currently, I only implemented it for Xcode projects, but I'll
file a followup task to ensure it's implemented for IntelliJ
projects as well.

Test Plan:
Unit test included. Ran on repository with a
directory which I added to project.default_exclude_paths.
Confirmed in logs that directory was excluded when I ran
'buck project'.
Fix the hash that works with Gradle in exopackage.html docs.

Summary:
https://twitter.com/hans_d/status/492021893026021376 rightfully pointed
out that the revision in our documentation does not build out of the box.

This is because AntennaPod requires you to create a config file:

https://github.com/danieloeh/AntennaPod/wiki/Building-AntennaPod#step-2-create-flattrconfigjava

We included a commit to create a dummy config so that AntennaPod builds out of the box:

https://github.com/dreiss/AntennaPod/commit/c2080b1dfd17fc371e04ce1e7b39ebadaf3cb7a7

But we forgot to use it as the base diff in our instructions (instead, we
were using the base of our fork, b2ae4c15d13840619fb09cc141b290449e40a6a4).

I verified that the build instructions in our docs work now. If there is a failure, you
likely need to run `$ANDROID_HOME/tools/android sdk` and install the
appropriate packages, which may include:

* Android SDK Build-tools 19.0.3
* Android Support Repository

Test Plan: Followed the updated build instructions and verified that they worked.