Update developer doc to reflect 'conda pip' is not allowed in v1.8.0

Developer documentation on using iPython notebook updated. Anaconda version 1.8.0 no longer supports 'conda pip' for installing packages in PyPI. Until a conda build package is created for music21, developers should us 'pip install' to install music21 when using Anaconda version 1.8.0 or higher. [see Issue 38]

Some formatting changes and cleanup done while working on this file to make it more user-friendly.