Center `+` and `-` signs of Graph.

Added CSS to center the zoom signs. Applied only to the JS/CSS version,
so not the the GWT version (if that is necessary).
Revert "Center `+` and `-` signs of Graph."

This reverts commit 5004769e5f06b37fe5f747e3ccc4e27e452e26c5.
Center zoom signs of Graph

Added CSS to center the zoom signs. Applied only to the JS/CSS version,
so not the the GWT version (if that is necessary).
Merge pull request #136 from rvlasveld/center-zoom-buttons

Center zoom buttons