backported modelASSIMP2

it's really the code from modelASSIMP3 with different include headers
indentation
indentation script

TODO: add 'modelines' for vim & emacs
documentation (and TODO)
renamed VertexBuffer.stride to VertexBuffer.dimen

as this is really the meaning of this member
adapt to member-rename of VertexBuffer.dimen
fix typo in comment (FTBFS)
moved VertexBuffer into namespace "gem"
use gem::VertexBuffer

it has moved into a namespace!
fixed typo: ambiant -> ambient
document that some methods in gem::VertexBuffer require a valid context
early return on error