Update authors and contributors list
Add initial CONTRIBUTING document

Based on
https://github.com/puppetlabs/puppet/blob/master/CONTRIBUTING.md

Ref #287
Remove contibuting info from docs
Added green_yellow_orange_red gradient for line % in solarized

Ref #301
Make green_yellow_orange_red grad use <16 colors, added 2 more grads

They are currently unused
Make it use gradient for weather temperature

Ref #301
Make generate_gradients tool be able to show custom scale
Made tools/generate_gradients.py work in python3
Made tools/*.py executable
Make prompt* sets non-local

Fixes #342
Merge remote-tracking branch 'zyx-i/fix-342' into develop
Autodetect network interfaces to monitor
Merge branch 'feature/autodetect-network-interfaces' into develop
Make weather gradient configurable, add system_load gradient

Also make gradients floating-point and fix #332 by removing “normalized” from 
system_load segment documentation.
Fix checks for gradient groups (it was not forcing gradient colors)
Added gradient for network_load segment
Make test.sh always check only powerline/config_files directory
Add gradient to email_imap_alert segment

Colors probably need to be revised

Fixes #301
Remove get_segment_info and passing segment_info through globals
Fix bold attr that may leak into next segment