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 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
Some lint fixes

* Fixed handling of empty scalars in python2: ''.join([]) returns str,
  ''.join([u'smth']) returns unicode
* Fixed check_config: it was always checking for themes, even if it was
  requested to check colorscheme
Added support for all powerline prompts (in2, out, rewrite)

Also fixed problem with prompt not really updating.

Fixes #141
Fix bytes handling on python3

Fixes #356
Autodetect python version