Add max results parameters to EC2 describe instances and describe tags calls. Also updates describe spot instance price history call with a max results parameter.
Merge pull request #1873 from danielgtaylor/ec2-max-results

Add max results parameters to EC2 describe instances and describe tags. Fixes #1873.
Version bump to 2.19.0 and added release notes.
Merge branch 'release-2.19.0' into develop.