Updates Python 3 install instructions, fixes #710
Updates Python 3 install instructions, fixes #710

Refactors Python 2/3 install instructions.
Makes explicit difference between host and target Python.
Both host Python versions are supported seamlessly while target Python
requires different actions depending on the version.

inclement/buildozer,kivy/buildozer,kivy/buildozer,rnixx/buildozer,kivy/buildozer,kivy/buildozer,inclement/buildozer,inclement/buildozer,rnixx/buildozer,inclement/buildozer,rnixx/buildozer,rnixx/buildozer