fix #79

I added the error suggested by Andres Almiray.

**WARNING:** This pull request contains changes which will most likely break some builds using an existing version of the plugin. 

In order to compare files by there location (i.e. must be located in a subdir of sourceDir) in the file system I was forced to change the comparison from just `file.name` to `file.canonicalPath`. This changes the behavior of the plugin.

Until now the plugin processes any file in sourceDir (or or any of its subdirectories) which matches the file **name** of a file given in the sourceDocumentNames. The location of the files in the sourceDocumentNames did not matter at all. They could be located anywhere.

This pull request changes this behavior. Now it is required for all files to be in sourceDir (or or any of its subdirectories). Also only the exact file specified in sourceDocumentNames will be included. **Any other file with the same name but located in a different subfolder will be ignored.**