Update categories.json

I added some categories:
general = Stuff like Tango or web.d with various stuff that doesn't fit into categories.
generic = Library code with focus on generic code, like parsers that don't focus on e.g. D code alone.
encoding = Conversion of charsets, Unicode standards, D strings, C strings, Pascal strings, ...
i18n = Anything that helps with internationalization and localization
scientific = Group of categories for math, physics, bioinformatics and chemistry.
optimized_cpu/_mem = Hint on libraries that are highly optimized for speed (possibly trading it for maintainability, safety or genericity) or memory use.
Merge pull request #28 from mleise/patch-1

Update categories.json