Add dynamodb policies

[ekrengel]

• set default params for elastic search policies
• add some rds policies
• add one dynamodb policy

Addresses some of policies in #11

[chrsmith]

Pushed a commit to fix the broken unit test. I was able to reproduce the Travis failure locally.

I would have guessed that the way to fix this would be with the following. (Passing a value to the throws function rather than the "constructor-like thing to the Error interface?)

            assert.throws(() => { database.rdsInstanceBackupEnabled("mandatory", 0); },
+                 new Error("Specified retention period must be greater than 0."));
-                 Error("Specified retention period must be greater than 0."));

But that didn't work either. (Not sure why, since from the docs that appears to be supported.) https://nodejs.org/api/assert.html#assert_assert_throws_fn_error_message

So I just changed it to just pass the expected error message. (Which is probably for the best, since verifying that the type of the thrown exception is Error doesn't really add much.)

[chrsmith]

@ekrengel merging so this doesn't become stale with pending SDK changes.