Detection of existence of key 'PATH' in os.environ

As evidenced by @simkimsia, running unoconv through nginx/php has reason to fail
 * This appears to be due to nginx not setting/including the PATH environment variable
 * Thus when unoconv attempts to read os.environ['PATH'], it can't because the key doesn't exist, causing a fatal error

The changes in this pull request remedy the problem by detecting if os.environ has the key 'PATH' and creating it if it does not.