Composer extension now pulls `ext-` requirements from the platform and
packages in composer.json and composer.lock.  These are converted to PHP
extension names by removing the `ext-` and are added to the list of
extensions that the build pack will install.

Extension version numbers from the composer files are ignored.  Build
pack always includes the latest version of an extension that was
available when PHP was compiled.
First draft of Composer docs.  More to come on this front.
Composer extension will now pull the PHP version out of `composer.json`
or `composer.lock` (`composer.json` is checked first).  If it's not
specified, it defaults to the version set by the build pack (i.e.
options.json).

The following syntax is supported for version numbers.

  - 5.3.* -> latest 5.4.x release (5.3, not supported)
  - >=5.3 -> latest 5.4.x release (5.3, not supported)
  - 5.4.* -> latest 5.4.x release
  - >=5.4 -> latest 5.4.x release
  - 5.5.* -> latest 5.5.x release
  - >=5.5 -> latest 5.5.x release
  - 5.4.31 (or some other specific release number) -> exact version
  - 5.5.15 (or some other specific release number) -> exact version
Added debug statement to list PHP version picked by composer extension.
Updated Composer docs with info about version.