Format the possible version formats as a table

Note I: This syntax is supported by MarkdownExtra, which both getcomposer
and github are using.

Note II: This patch currently breaks the PDF, because pandoc does not like
non-standard markdown tables. Ideas for fixing this appreciated.

Note III: The idea for this patch came up a few weeks ago on IRC. We
agreed back then that a table would be a good idea.

Note IIII: This patch creates a stability section which opens the door for
finally documenting how stability works in the composer docs.
Document logical OR constraints
Allow specifying HHVM as a dependency
hhvm version constant still called HPHP on older versions so use it for bc
Check only part of the error message as it's different in hhvm
Fix stream context option test to really only verify content-type is last
Merge pull request #2302 from igorw/versions-table

Format the possible version formats as a table
Change dependency processing for update with whitelisted packages
By default dependencies of white listed packages are no longer installed automaticaly.
To Install dependencies of whitelisted packages use --with-dependencies.
- rework after review comments
- precise documentation of option
- add missing punctuation marks