Fixed mininet.org url in README.md.

s/http:\/\/openflow.org\/mininet/http:\/\/mininet.org/g
mnexec.c: substituting tabs with spaces.
Adding mininet doxygen installation dependencies.

Right now after installing mininet using util/install.sh there is a problem when try to generate doc:
	make doc
	doxygen doc/doxygen.cfg
	make: doxygen: Command not found
	make: *** [doc] Error 127
Adding doxygen, doxypy and texlive-fonts-recommended packages to fix the problem.
Merge pull request #171 from vitalivanov/dev

Cleanup and cosmetic changes.