Added the functions from Graphs.jl

And a documentation folder
Stupid typo
Added Distributions when loading the module
Does it runs now?
Changed the prototype of niche_model_graph
Removed niche model / does it builds now?
Indentation fix
Tests in the niche model function
Probably found the bug
Merge pull request #1 from tpoisot/master

Added a bunch of models and some documentation.