bump version to 0.22.0

Merge pull request google#256 from cbracken/matcher_imports

Remove redundant matcher imports.

Version 0.21.3+1

Version 0.21.3

Version 0.21.2

Version 0.21.1

Merge pull request google#211 from cbracken/deprecated_strings_apis

Document that padLeft/Right, trimLeft/Right will be removed in 0.22.0.

Fix hanging FakeAsync test.

Add upper bound to matcher dependency.