Set $COMPOSER_HOME into the virtphp env path

Setting $COMPOSER_HOME to $VIRTPHP_ENV_PATH/.composer, it would make all `composer global xxx` be executed in the virtphp env path, instead of user's $HOME.
Applied @ramsey patch with some fixes
Composer folder for the env should not be hidden
Merge pull request #37 from chadrien/patch-1

Set $COMPOSER_HOME into the virtphp env path