ScriptService: add getScript(String,String) signature

[imagejan]

This makes it easier to retrieve a ScriptInfo object from a script string (without going via File).

Closes #389.

[imagejan]

I refactored ScriptServiceTest a bit to initialize the Context in a @Before method, and dispose it @After the tests.

@ctrueden what do you think about the re-use of the ScriptLanguage from ScriptInfoTest$BindingSizes? Should we instead make BindingSizes its own class within the tests? There are actually two nested ScriptLanguage implementations in the tests currently: ScriptInfoTest$BindingSizes and ScriptEngineTest$Rot13.