Fix finding a composer executable

[kaloyan-raev]

This patch makes the following improvements for finding a composer
executable:

1. The check if composer is available on the executable path should
   check the exit code instead of the output. The assumption that the
   output will be empty if there is no executable is wrong. Checking the
   exit code is more reliable.
2. The usage of file_put_contents() to download composer.phar was wrong.
   Instead of downloading the composer.phar, a text file with the download
   URL was created. This is now fixed.
3. The patch makes sure that the downloaded composer.phar will be
   executable on UNIX systems.
4. The getComposerExecutable() method now returns the full path of the
   downloaded or existing composer.phar, which makes it executable on UNIX
   systems too. Just 'composer.phar' cannot execute the file from the
   current directory. It must be at least './composer.phar'.
5. The error message if composer fails now includes the exit code and
   the last line of the output.

[ezimuel]

Thanks for the PR!