Update documentation
Rename `is_watched` method to `is_watching`
Add tests for is_watching method

Currently they fail for uv-based watcher.
Add `is_watching` method to UvWatcher class

Fixes #1052
Add realpath() calls to Uv watchers where appropriate
Return False in place of None to indicate absense of changes

Fixes #1052: another part of it
Merge pull request #1053 from ZyX-I/fix-uv-watcher

Some fixes for libuv-based watcher