BibIndex: Greek stemmer improvements

• Optimizes the greek stemmer class by pre-compiling all the regular expression patterns and other helper variables declaring them as private constants. (closes #908)

• Changes the Greek stemmer class definition to the new-style class, inheriting object, and changes its name to GreekStemmer to match Invenio's classes naming convention.

• Changes the name of the main stem function from stem_word to stemWord to match SnowBall PyStemmer's respective stem function.

• Adds the function stemWords which accepts a list of words and returns a list of their stems, matching SnowBall PyStemmer's respective stemWords function.

• Adds a function which replaces accented vowels with their non-accented versions and another function which converts all the cased characters into uppercase. These two functions together prepare any given word for the main stem function.