Grid level alignment. Deleting grid items and tracks.

New

#54 Add UI to delete selected cell or track.
#53 Add UI to control grid level alignment. enhancement
Added credits

Fixes

#52 It is impossible to go back in browser.

Improved UI for grid configuration

Powergrid v0.4 brings you a better UI to configure your grid. Here are some interesting features from this release,

• Enables use of images instead of text in the configuration
• Moved various settings options into a single configuration modal
• UI is now driven through URL parameters, so you can easily share your configuration
• Better documentation

UI for Configuration

Update CONTRIBUTING.md

Initial commit.