BibTeX “abstract” field / PHPUnit default config file

I made two minor changes/additions:

1. Added the missing “abstract” in BibTeX\Parser::create(). (A client needs access to abstracts from a BibTeX file.)
2. Added a default PHPUnit config file. This way, one can simply “cd” into the “tests” directory and execute “phpunit” without even thinking about the bootstrap file or where the tests are.