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
disable prepend option on install
disable prepend option on update
Add local cache for Git repositories
Add temp composer home for GitDriver test using cache
added warning about referencing dependencies in pre-install and pre-update
fix up markdown
fix line lengths
Fix link to COMPOSER_HOME
Merge pull request #2323 from yuya-takeyama/patch-1

Fix link to COMPOSER_HOME on doc
Fix for Preforce utility does not check if p4 command exists
Check for exit code is equal to 0 instead of 127
Changes comparsion to strict and removed a comment
Removed unnecessary test and just check for return code
refactor prepend autoloader from cli option to config var (prepend-autoloader) in composer.json
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