Make library.properties compliant with Arduino library specification
Make library.properties compliant with Arduino library specification

- Add URL field, without which installation fails silently.
- Add category fiels, without which a warning is displayed on every
compilation in Arduino IDE 1.6.6 or later:
WARNING: Category '' in library Neotimer is not valid. Setting to
'Uncategorized'
- Use correct architectures field syntax.

jrullan/neotimer