use ES6 export, export all directives as default export, add default export test

[Saeris]

Updated all files (except for src/directiveLocation) to use ES6 export syntax.

Changed the default export for the library to be an array of all directives to streamline usage in situations where a user wants to use all of the directives provided by the library.

Added a new test to ensure the default export is an array containing only instances of GraphQLDirective.

Updated README examples with new usage.

[coveralls]

Coverage remained the same at 100.0% when pulling 8d6ba5a928babd65c42c9761252ad056aa245903 on Saeris:export-all-directives into 8ad5e70351343fa3374f80ac3335e472febaa42d on lirown:master.

[lirown]

conflicts?

[Saeris]

Conflicts should be resolved