Fixes to conform to MLA standards

MLA format requires that the hanging indent of citations be 0.5in. This is not the default in biblatex-mla. This commit sets \bibhang appropriately.

Also, commit https://github.com/kc9jud/mla13/commit/b6187f5255258e132678d6ad5dac43c8cc64efd2 fixes the missing parentheses as discussed in https://github.com/jmclawson/biblatex-mla/issues/5