i18n: add greek language
i18n: greek language update

Added month names nominative and genitive handling. The rule is that if a day
number (D) exists in the format before MMMM then genitive is used or else we
use nominative.

examples:
"Do MMMM YYYY"       should be like "19η Φεβρουαρίου 2013"
"dddd, DD MMMM YYYY" should be like "Τρίτη, 19 Φεβρουαρίου 2013"
"MMMM YYYY"          should be like "Φεβρουάριος 2013"
"dddd, MMMM YYYY"    should be like "Τρίτη, Φεβρουαρίος 2013"
Merge pull request #640 from mehiel/feature/el-lang

i18n: add greek language