feat(method): proxy to VK.api when available

Proxy API calls to the official VK JavaScript SDK when possible.

Related to #1.

src/vk.coffee

@@ -77,22 +77,27 @@ module.exports =
 
 
   method: ( methodName, params = {}, callback = -> ) ->
-    params.access_token = @accessToken
-    params.v = @version
-
     new Promise ( resolve, reject ) =>
       do retry = =>
-        @request
-          method: "POST"
-          url: "https://api.vk.com/method/#{methodName}"
-          params: params
-          callback: ({ error, response }) ->
-            if error?
-              if error.error_code is ERROR_TOO_MANY_REQUESTS
-                setTimeout retry, 300
-              else
-                callback error, response
-                reject error
+        wrappedCallback = ({ error, response }) ->
+          if error?
+            if error.error_code is ERROR_TOO_MANY_REQUESTS
+              setTimeout retry, 300
             else
               callback error, response
-              resolve response
+              reject error
+          else
+            callback error, response
+            resolve response
+
+        if window.VK?.api?
+          window.VK.api methodName, params, wrappedCallback
+
+        else
+          params.access_token = @accessToken
+          params.v = @version
+          @request
+            method: "POST"
+            url: "https://api.vk.com/method/#{methodName}"
+            params: params
+            callback: wrappedCallback

test/vk.coffee

@@ -237,6 +237,7 @@ describe "vk", ->
     fakeUrl = "https://api.vk.com/method/fake-method"
 
     beforeEach ->
+      delete window.VK
       vk.accessToken = "fake-token"
       vk.version = "fake-version"
 
@@ -335,6 +336,30 @@ describe "vk", ->
         done new Error "rejected!"
 
 
+    it "should proxy calls to VK.api when possible", ( done ) ->
+      fakeData =
+        error: "fake error"
+        response:
+          foo: "bar"
+
+      window.VK =
+        api: ( method, params, callback ) ->
+          method.should.equal fakeMethod
+          params.should.deep.equal
+            foo: "bar"
+
+          callback fakeData
+
+      vk.request = ->
+        done new Error "called vk.request!"
+
+      vk.method fakeMethod, foo: "bar", ( error, response ) ->
+        expect( error ).to.equal fakeData.error
+        response.should.equal fakeData.response
+
+        done()
+
+
   describe "request", ->
 
     fakeUrl = "https://api.vk.com/method/fake-method"