* Enable declarationMap in tsconfig.json
See http://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-9.html
* Add apollo-server-caching package and improve typings
* Remove superfluous test steps
* Add .npmignore to apollo-server-caching
* Add apollo-server-env and apollo-datasource-rest packages
* Fix broken imports
* Use prepublish instead of prepare
* cache is now passed to data sources from ApolloServer constructor
* fix Object.values to use the object passed in rather than this
* add initial datasource test
* docs: initial data source documentation
* docs: initial data source documentation
* compiles and documentation now highlights code in data-sources.md
* Some changes to the data source docs
* Add BadUserInputError and TransientError as extensions of ApolloError
* remove TransientError
* update test so that properties end up under `error.extensions.exception`
* update documentation to talk about BadUserInputError
* check for properties after formatApolloErrors has been called
* fixed highlighted lines for docs
* export BadUserInputError from `apollo-server`
* fixed highlighted lines for docs
* export BadUserInputError from `apollo-server`
* update changelog
* docs: copy initial stitching sections from graphql-tools
* docs: convert schema stitching introduction to use apollo-server
* docs: remote-schema shortened to contain links only
* docs: schema delegation shortened to remove api reference
* docs: rename schema-stitching to features
