Merge pull request #23 from mapzen/gk

Gk
Added tests for addition operators
Added unit tests for Subtraction Operators
Added unit tests for Multiplication Operators
Added unit tests for Equal To Operator
Add tiled graph structures.
Merge pull request #24 from mapzen/gdg

Added unit tests for Addition, Subtraction, Multiplication, and Equal To Operators
Fix GraphId constructor
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.
Remove data.
Update comment
Fix derived constructor to call base class constructor
Merge pull request #27 from mapzen/dwn

Add initial "builder" classes for tiled graph
Fixing the makefile and compile issues from last PR
Merge pull request #28 from mapzen/dwn

Fixes to DirectedEdgeBuilder
Added GraphTileHeaderBuilder