Cleanup old incubator references left behind.

Couple references to when the project was in the incubator are still
in the codebase, removing and updating pom to point to correct mailing
list.

From: Jake Farrell <jfarrell@apache.org>
Review: https://reviews.apache.org/r/17032
Fixed a bug in the log recover test.

A wrong quorum size was used.

From: Jie Yu <yujie.jay@gmail.com>
Review: https://reviews.apache.org/r/17036
Added a template for .gitignore.

Contains the files and directories that are generated by running
./bootstrap.

It also masks out .gitignore and the recommended ./build directory.

To use run:
  cp .gitignore-template .gitignore

From: Charlie Carson <ccarson@twitter.com>
Review: https://reviews.apache.org/r/17001
Fixed the flaky GroupTest.RetryableErrors test.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/16824
Cleanup of stout headers from fedora review.

From: Timothy St. Clair <tstclair@redhat.com>
Review: https://reviews.apache.org/r/16830
Add post function to http.hpp of libprocess.

This adds the post equivalent of the get function to http.hpp.

The existing get method is refactored into internal::request with
arguments for method, path, query, contentType, and body.

The get and post functions can then be implemented as simple wrappers
of internal::request.

The new post function has all required arguments of path, contentType,
and body.

The get function's query argument is also changed to be Option<string>
to better indicate that it is optional.

That change requires adding a forwarding overload with the old
signature (but w/o the default parameter value) for back-compat.

There are also new unit tests to verify the existing behavior of get
and the new post behavior.

From: Charlie Carson <ccarson@twitter.com>
Review: https://reviews.apache.org/r/16839