Fixes #86

[build] Simplify recipe parsing and environment creation significantly and fixes multiple environment creation issues; Closes #70

[build,scripts,data] Rename nosetests specific mark expression to be more generic and enable pytest equivalent

[scripts/create] Implement --pip-extras option to allow the user to install extra pip-installable packages on every created environment

[build,scripts,data] Allows the environment variable `$NOSE_EVAL_ATTR` to indicate which tests to skip during conda build (c.f. bob/bob.nightlies#55)

This allows to test bootstrapping the environment with new packages
without the need to first have them integrated in bob.conda.