Promise support for Nano - Issue 98 (#102)

* package-lock.json audit fix

* return Promise by default

* tests passing

* amend integration/document tests wrt Promise return values

* amend integration/attachment tests wrt Promise return values

* amend integration/database tests wrt Promise return values

* amend integration/design tests wrt Promise return values

* amend integration/multipart tests wrt Promise return values

* amend integration/shared tests wrt Promise return values

* amend integration/util tests wrt Promise return values

* amend intercept/design tests wrt Promise return values

* readme rewrite

* first stab at TypeScript definition returning Promises

* added ...AsStream varaiants

* add TypeScript to the README

* test fix

* find query missing from README

* added findAsStream and find tests

* added searchAsStream

* searchAsStream README

* add viewAsStream

* added changesAsStream

* fix tests

* add migration guide from nano 6 to nano 7

* tests and jshint work

* 99+% test coverage

* migration guide

* swap out var for const/let in the core code and tests

* refactor responseHandler to make parameter names more readable

* add default parameter value in responseHandler

* return headers for headDoc on an error

* added central getCallback function to remove code duplication

* var--->const in the README

* modernise JS in migration guide

* jshint fixes

* further documentation changes after feedback from @janl

* use official CouchDB docker image at the suggestion of @wohali

Author: glynnbird

.jshintrc

@@ -3,11 +3,12 @@
   "node":      true,
   "strict":    true,
   "smarttabs": true,
-  "maxlen":    80,
+  "maxlen":    100,
   "newcap":    false,
   "undef":     true,
   "unused":    true,
   "onecase":   true,
   "indent":    2,
-  "sub":       true
+  "sub":       true,
+  "esnext":    true
 }