Create osap.csl

Oxford Studies in Ancient Philosophy citation style.
Update osap.csl

Updated as requested according to 3,4,6, 8, and 9 here: https://github.com/citation-style-language/styles/wiki/Style-Requirements
Update osap.csl

Minor correction.
Update and rename osap.csl to oxford-studies-in-ancient-philosophy.csl

Corrections.
Merge pull request #696 from jamiedow/patch-1

Create osap.csl