GROOVY-3457: Fixing bug where line number of template error was incorrectly reported

o Added more granular unit tests correct line numbers on
  exceptions thrown when parsing/executing a template
o Added context reporting so that three context lines of code is
  now added to the thrown template errors even if it is a template
  parse (vs execution) error.
o Added and fixed various javadocs for StreamingTemplateEngine
GROOVY-3457: Cleaning white space
GROOVY-3457: Fixing javadocs for StreamingTemplateEngine
Merge pull request #534 from mbjarland/master

GROOVY-3457: Fixing javadocs and unit tests for StreamingTemplateEngine