Finalizing Pandana v0.5 release

[smmaurer]

This PR finalizes the v0.5 release by merging dev into master.

• [x] Released on Pip: https://pypi.org/project/pandana/#files
• [x] Released on Conda: https://anaconda.org/conda-forge/pandana/files
• [x] Documentation updated: http://udst.github.io/pandana/

Change log for v0.5:

• Adds support for calculating shortest path lengths between arbitrary origins and destinations, with vectorization and multi-threading (PR #134, #136)
• Restores alternate names for aggregation types, which were inadvertently removed in v0.4 (PR #140)
• Fixes a bug with matplotlib backends (PR #135)
• Improves compilation in MacOS 10.15 Catalina (PR #137)
• Eliminates the scikit-learn dependency (PR #140)
• Makes matplotlib and osmnet dependencies optional (PR #140)
• Revises the documentation and demo notebook (PR #140: Pandana-demo.ipynb)

Tagging folks who have contributed or been active in the issue discussions -- thanks!

@sablanchard @jessicacamacho @federicofernandez @knaaptime @jreades @thinkersloo