Ignore packages symlinks
Add Hop task to run unit tests

From the Stitch folder, run `dart tool/hop_runner.dart test` to execute all unit
tests for the package. Run `dart tool/hop_runner.dart help test` for more
information about this task.

Closes #5.
Clean up method to build testable HtmlInspector transformer
Add static method to generate a class name for a sprite
Test that paths are mapped properly for assets in the web directory
Test content for generated CSS helper
Add tests for sprite sheet
Fix typo in readme
Bump version to 0.1.0+2