Add pull request filter by head and base

According to https://developer.github.com/v3/pulls/#list-pull-requests
we can now filter PR by head and base.

Author: jillro

api/v3.0.0/pullRequests.js

@@ -30,6 +30,8 @@ var pullRequests = module.exports = {
      *  - user (String): Required. 
      *  - repo (String): Required. 
      *  - state (String): Optional. open, closed, or all Validation rule: ` ^(open|closed|all)$ `.
+     *  - head (String): Optional. Filter pulls by head user and branch name in the format of user:ref-name. Example: github:new-script-format.
+     *  - base (String): Optional. Filter pulls by base branch name. Example: gh-pages.
      *  - page (Number): Optional. Page number of the results to fetch. Validation rule: ` ^[0-9]+$ `.
      *  - per_page (Number): Optional. A custom page size up to 100. Default is 30. Validation rule: ` ^[0-9]+$ `.
      *  - sort (String): Optional. Possible values are: `created`, `updated`, `popularity`, `long-running`, Default: `created` Validation rule: ` ^(created|updated|popularity|long-running)$ `.

api/v3.0.0/pullRequestsTest.js

@@ -31,6 +31,8 @@ describe("[pullRequests]", function() {
             {
                 user: "String",
                 repo: "String",
+                base: "String",
+                head: "String",
                 state: "String",
                 page: "Number",
                 per_page: "Number"

api/v3.0.0/routes.json

@@ -1617,6 +1617,8 @@
                     "invalidmsg": "open, closed, all, default: open",
                     "description": "open, closed, or all"
                 },
+                "$head": null,
+                "$base": null,
                 "$page": null,
                 "$per_page": null,
                 "sort": {