mime types: add magic pattern to the circuit file type

this fixes the mimetype identification of circuit files on systems where the .circuit filetype is defined in multiple mime databases.
the beginning of the file is verified for a matching header, so the .circuit file won't be text/plain anymore.
Merge pull request #22 from ktechlab/fix-circuit-opening-v2

mime types: add magic pattern to the circuit file type and fix the circuit-opened-as-text problem