Add tests for DiagnosticsEngine and Diagnostic.
Use modern minitest from rubygems

The minitest shipped with ruby does not have all assertions.
Add SourceFile#{position_to_line,line}.
Don't return line endings in SourceFile#line.
Add Diagnostic#render.
Minor diagnostic-related tweaks.
Add Range#size shim for 1.9.3.
Add optinal parentheses for method calls with arguments
Use single quotes for non interpolated string literals
Fix 1.9 specific Range#size hack

Enumerator does not have #size eighter.