Fix for issue #23 - source maps should use relative paths to resources included from webjars

 - included assets absolute paths are now replaced with '../' prefix in writeSourceMap() handler
 - added relativeImports option to turn new functionality ON and OFF (by default it is ON)
 - added two tests for relativeImports option
Merge pull request #44 from Blobson/webjars-paths

Fix for issue #23 - source maps should use relative paths to resources i...