Plugin no longer passes plugin-only options to Composer. (Fixes #8)
Distinguish between Composer downloaded or error

In _checkComposerPhar(), make error message distinguish between error executing composer.phar and if it exists at all.

On my host, phar was not added to suhosin whitelist, so composer.phar wouldn't run. cakephp-composer only said that Composer wasn't installed, but the error was actually that composer failed to run.
Merge pull request #9 from eithe/master

Distinguish between Composer downloaded or error
Fix executing composer with spaces in path
Added getComposerVersion phpdoc
Merge pull request #11 from dmromanov/master

Fix executing composer with spaces in path
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