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]
Add enable_agent option to puppet plugin
Move "sections" setting into "packages" section.
Rename it to "components", which is the real name for it (see http://manpages.debian.org/cgi-bin/man.cgi?query=sources.list).
Fix minor mistakes in log_call
Change order of expected output in log_call test
This will fail on OSX but work on Debian (no idea as to why, yet)
Use string concatenation instead of format()
String concatenation can at times be easier to read that format().
One should choose whichever approach is more readable.
Make the gpt_offset fiddling easier to understand
Fix AdjustExpandRootScript
Also, only add it when it is required
Fix wrong bootloader vs. virtualization check