add '$ make doc' with files in assets now properly licensed

correct mistake in the Makefile, clean enhanced doc when calling make clean, add enhanced doc to gitignore

rename directory

move enhanced-doc directory to directory util, adapt Makefile
fixes sed script in that if 'Proof.' is missing then corresponding 'Defined' or 'Qed' is ignored; fix by Guillaume Verdier
Merge pull request #44 from benediktahrens/e-coqdoc

add '$ make doc' with files in assets now properly licensed