Added tests for addition operators
Added unit tests for Subtraction Operators
Added unit tests for Multiplication Operators
Added unit tests for Equal To Operator
Merge pull request #24 from mapzen/gdg

Added unit tests for Addition, Subtraction, Multiplication, and Equal To Operators
Added C++ style guide link
Merge branch 'master' of https://github.com/mapzen/valhalla into gdg
Merge pull request #26 from mapzen/gdg

Added C++ style guide link
Merge pull request #25 from mapzen/dwn

Add tiled graph structures.
Add initial "builder" for Mjolnir. Update DirectedEdge formatting
and style.