Update README.md

Fixed file paths in "Use cases" section so that they correspond with the examples.
Add support for async attr in script blocks
tests for async blocks
useminPrepare accepts array of root directories
Fix indentation according to JSHint
Remove blocks if there are no linked assets
Lock Grunt to 0.4.1

"~0.4.1" was using a recently released 0.4.3, which introduced a
breaking change where tasks were executed asynchronously. This caused
tests to fail because assertions were executed before tasks finished
doing their thing.
Merge pull request #314 from silvenon/empty

Remove blocks if there are no linked assets
Merge pull request #269 from DexterV/master

Update README.md
Merge pull request #272 from sergiolopes/master

Add support for async attr in script blocks
update grunt dependency
Merge pull request #315 from tkellen/dep

update grunt dependency
Merge pull request #296 from donaldpipowitch/root-with-array

useminPrepare accepts array of root directories
Add missing space for consistency
Fix breaking tests
Bump to 2.1.0
Update README.md

Fix typo, syntax error and example consistency.
Merge pull request #321 from granteagon/patch-1

Update README.md
Merge with upstream