Update imaging.rst
Merge pull request #475 from evanv/master

Update imaging.rst
Added more info in virtualenvs.rst.
Added commands to specify python interpreter version.
Added build/ directory to .gitignore
Update learning.rst

Python Cookbook has been added to the list.
Merge pull request #477 from ugurcaliskan/patch-1

Update learning.rst
Merge pull request #476 from BastiTee/master

Added build/ directory to .gitignore
Merge pull request #474 from syndbg/master

Added more info in virtualenvs.rst.