Add tests/isolator.hpp to the distribution

Review: https://reviews.apache.org/r/20872
Fixed infinite recursion in test containerizer.

Review: https://reviews.apache.org/r/20870
Fixed distclean break during maven clean up.

The pom file gets cleaned up before clean-java runs, which calls maven
and fails. Solution is to just remove the target dir, which is what
maven does as well.

Review: https://reviews.apache.org/r/20874
Fixed some indentation.
Better error message for protobuf::write.

Review: https://reviews.apache.org/r/20072
Added 'mesos-usage' for use by external containerizers.

Review: https://reviews.apache.org/r/20070
Added test for slave stopping before containerizer launches.

Review: https://reviews.apache.org/r/20179
Allowed passing headers to http::get/post.

Review: https://reviews.apache.org/r/20275
Updated uses of http::post.

Review: https://reviews.apache.org/r/20278
Used memory::shared_ptr instead of boost::shared_ptr.

Review: https://reviews.apache.org/r/20305
Added 'cref' and 'ref' to lambda:: namespace.

Review: https://reviews.apache.org/r/20306
Added process::async overloads for void functions.

Also used preprocessor macros to enumerate overloads.

Review: https://reviews.apache.org/r/20307