Improve POT file generator

[sirzooro]

Please implement following enhancements for POT file generator:

• use UTF-8 enconfig. Now it removes all non-ascii characters, at least in author's name (you can check this with ​my plugins);
  
• read plugin name and put it in Project-Id-Version header (maybe version too);
  
• set charset in Content-Type header to UTF-8.

[nbachiyski]

All done.

The problem with author's name was caused by escapeshellarg(), which trims non-ASCII characters if the system locale encoding isn't UTF-8.

I also made the code, which set the project name and versions not depend on newer version of xgettext.