Added .gitattributes and fixed line feeds

In order to avoid trouble in future I've added a `.gitattribute` file which defines the line endings for the repository. Regarding the Mono compatibility of the project this would lead to an issue sooner than later. 

After that I've normalized the line endings. This affected 15 files. Therefore they look like they have completely changed - the content is the same.

See also http://www.lhotka.net/weblog/AdventuresInGitland.aspx

What do you think?