Added 'thirdparty' subfolder to the /docs folder to contain license files for 3rd party libraries.

Added miglayout-license.txt, taken from the miglayout 4.0 download even though we use 4.2, since the 4.2 download doesn't appear to include a license.txt
Issue: MIRTH-1699
Added copyright header to LICENSE.txt
Issue: MIRTH-1713