Centralize graph fn optional arg parsing.

ST_GraphAnalysis, ST_ShortestPathLength and ST_Accessibility all
contained parseOptionalArguments methods. This puts them all in the
GraphFunctionParser.
Improve parser and add more tests.
Replace .equals("") by .isEmpty()
Merge pull request #59 from agouge/refactor/graph-fn-parser

Refactor: Centralize optional argument parsing for graph functions