Clean up the "hello world" code sample (#328)

Changes to code:
+ Add region tags for including code in docs
+ Change `require()` call for consistency with the API docs; the sample now requires the library as `Bigtable` and creates an instance of it by calling `new Bigtable()`
+ Add comments about shortcuts that would cause performance issues in production
+ Fix misspelled variable

Changes to `README`:
+ Fix branding issues
+ Update auth instructions to use the current best practice; `gcloud auth application-default login` is no longer recommended
+ Update links to the GCP Console
+ Remove embedded screenshot from the docs, to prevent a broken image if that screenshot goes away
+ Miscellaneous changes for clarity and consistency

Author: hegemonic

handwritten/bigtable/CONTRIBUTORS

@@ -7,6 +7,7 @@ Alexander Fenster <github@fenster.name>
 Dave Gramlich <callmehiphop@gmail.com>
 Eric Uldall <ericuldall@gmail.com>
 Jason Dobry <jdobry@google.com>
+Jeff Williams <jefesaurus@google.com>
 Jiren Patel <jirenpatel@gmail.com>
 Jun Mukai <jun.mukai@gmail.com>
 Luke Sneeringer <lukesneeringer@google.com>