Merge pull request #123 from facetoe/master

Added method for printing a graph of Unicode characters.
Added method to generate a list of hex color values between a start
color and end color.
Added methods for retrieving network information to make the class
easier to extend.
Added module to draw a network graph in Unicode.
Moved get_hex_color_range() from util.py to network_graph.py to prevent
breaking the build.
Moved color code into it's own class that other classes can inherit
from.
Modified NetworkGraph to use new ColorRangeModule class.
Added module for graphing cpu usage.
Added color module to i3pystatus.rst.
Update CI configuration: test install and test building docs.
Update requirements.txt
Update .travis.yml
Documented dependencies on the PyPI colour module.
Update .travis.yml
Merge pull request #125 from facetoe/cpu_usage_graph

Cpu usage graph
Update util.py