alias extractor should not match use in anonymous functions

I adjusted the AliasExtractor so it does not match agains the use keyword in the context of anonymous functions
- this should fix #14
- additionally this fixes some wrong alias entries for classes with such use statements where the whitespace was set between use and (
- added test cases for all 4 scenarios I could think off

I wasn't sure how you would the null value handling. If you prefer it otherwise just let me know.