add typings reference in package.json
add typings reference in package.json

Update `package.json` to include a `typings` reference to the type definition making this package more typings friendly. See #152, specifically this comment https://github.com/Azure/azure-storage-node/issues/152#issuecomment-235195913 for reference.

XiaoningLiu/azure-storage-node,XiaoningLiu/azure-storage-node,Azure/azure-storage-node,Azure/azure-storage-node