Update README.md

typos
Update README.md
Start using polymer spark-menu-button
remove Spark singleton
Merge pull request #430 from dart-lang/fix_polymer

remove Spark singleton
Merge pull request #377 from andersoyvind/master

Update README.md
Merge pull request #428 from dart-lang/new_menus

Start using polymer spark-menu-button
Small style fixes

https://github.com/dart-lang/spark/pull/424
Merge pull request #433 from ussuri/small-style-fixes

Small style fixes
Merged