Add `Save` method to `Request`

[matryer]

Save should:

• Do a Create if the resource has no ID
• Do an Update if the resource has no ID

OR

We could have Create, Update, and Replace as their own methods and do the Save helper later.

[matryer]

see https://github.com/stretchrcom/stretchr-sdk-go/issues/12 first

• The Save method could in fact be placed on the Resource.

[tylerstillwater]

We don't need to do an ID check, as a PUT will create it anyway.