Preparing v0.2.1 release

[smmaurer]

This PR prepares the v0.2.1 release.

Previously merged:

• Support for GeoPy 2.0+ (PR Geopy fix #62)
• Support for Pandas 1.0+ (PR as_matrix deprecated #65)

This PR:

• adds some unit tests (@sablanchard)
• updates version numbers
• updates change log
• cleans up setup script
• cleans up Travis script
• cleans up docs
• adds contribution/release guide
• minor updates to README
• updates demo notebook to plot Pandana network results using Matplotlib and CartoPy instead of Pandana's plot function (@sablanchard)
• minor update to a print statement
• adds requirements-dev.txt
• updates to pycodestyle usage

Before merging:

• resolve OSMnet version error in py27 tests (https://travis-ci.org/github/UDST/urbanaccess)
• resolve Pandas column order error in py35 tests
• resolve Matplotlib issues in tests

After merging:

• release on pypi
• release on conda forge
• update doc website
• merge to master and tag in GitHub