Add contribution guidelines

I've added a _Contributing_ section to the README and a [CONTRIBUTING
file](https://github.com/blog/1184-contributing-guidelines). I do not
expected for these changes to be merged right away, as they can be seen
as a draft from what I've observed in the past weeks. The intention here
is not only help the developers that are new to the project, but also
make it easier for Anders to deal with new patches.
Add a note about notification on pull requests
Add a note about rebasing pull requests
Merge pull request #68 from myhro/new-contributing

Add contribution guidelines
Fix some typos on the CONTRIBUTING file
Add info about gh-pages-dev on CONTRIBUTING
Add missing unstable configuration fixing EC2 image building problems.
Merge pull request #77 from rybaktomasz/fix-upstream

Add missing unstable configuration fixing EC2 image building problems.
Fix SetMetadataSource predecessors

Added GenerateLocale as SetMetadataSource predecessor, to avoid any
potential locale warnings. Also fixed a space indentation flaw and a
string which was being formatted with a non-existent key-argument.

This closes #75.
Add Travis CI

By now, it will only test the coding style rules defined for the
project, which are basically a few exceptions to the PEP8.
PEP8 fixes
Add information about coding style checking

Added a note about this to the CONTRIBUTING.md file. [skip ci]
Added manifest checking tests.

Overview:
1) fixed up .gitignore file to use absolute paths (to project dir).
2) updated tox to run both manifest tests and flake8
3) updated yml file cause NULL for aws credentials causes an error on validation.
4) actually added the tests under tests/integration
Merge pull request #79 from Rory-Finnegan/testing

Testing
Add Google Cloud Engine (GCE) provider:

 * force kpartx to use synchronous mode, removing sleep(10)
 * get image configuration, use it during tarball creation and registration
 * add (non-working, path problems) image registration
 * add cleaning of image from Google keys and repositories
 * add NTP server address in manifest
 * add preference for backport kernels in manifest
 * disable IPv6
 * correctly set host name
Merge pull request #80 from rybaktomasz/gce_development

Add Google Cloud Engine (GCE) provider:
Fix import error with cloud_init task
Fixes #14 and #78
S3 images can now be bootstrapped outside EC2.
However, this fix adds the requirement of a "region" setting.
At some point we could make "region" optional and fail when bootstrap-vz
is run outside EC2 and no "region" is set.
Require hostname setting for vagrant plugin
Also, fix reference to vbox src to a revision
Simplify adding tasks
There is now a get_standard_groups function