Add xwalk extension system to cordova-xwalk-android.

Changes in this patch:
 1. Implement XWalkRuntimeViewProvider interface for CordovaWebView.
 2. Add a public class CordovaXWalkCoreExtensionBridge to provide the
    functionalities of friendly class XWalkCoreExtensionBridge.
Merge pull request #57 from gaochun/extension_system

Add xwalk extension system to cordova-xwalk-android.
Add empty implementation for loadDataForTest in CordovaWebView

Otherwise, build is blocked for abstract function not implemented.
Merge pull request #62 from wang16/xwalk

Add empty implementation for loadDataForTest in CordovaWebView
Implement the destroyExtension interface

It is introduced in XWalkRuntimeViewProvider to destroy the native
resources occupied by extension.
Merge pull request #63 from hmin/destroyExtension

Implement the destroyExtension interface
Merge pull request #61 from huningxin/readme

Update the README.md according to Crosswalk dependency.
Use XWalkExtensionManager to reuse the extension system

This is a refactor to remove unnecessary interfaces exposed by
XWalkRuntimeViewProvider. Reusing XWalkExtensionManager is enough to
integrate the extension system into cordova container.
[Android] Refine the test cases for thread safety checking.

Schedule the webview's API called on UI thread, otherwise, that
would cause the RuntimeException. This patch refines two test case
of IFrameTest, one test case of HtmlNotFoundTest and one test case
of CordovaActivityTest.

BUG=https://github.com/otcshare/cordova-xwalk-android/issues/52
Merge pull request #73 from junmin-zhu/test_case_fix_1

[Android] Refine the test cases for thread safety checking.
Execute `android update` with host supported target.

It avoids to use unsupported target on developers' system.
Enable bin/create.js on Windows to create apps

The additional steps are the same as the ones made on
bin/create for Linux.
Only difference is that on Windows, the fullname of python
need to be detected by "where". It could be python.bat or
python.exe.

BUG=https://github.com/otcshare/cordova-xwalk-android/issues/34
Merge pull request #75 from huningxin/fix_target

Execute `android update` with host supported target.
Merge pull request #72 from hmin/extension-refactor

Use XWalkExtensionManager to reuse the extension system
Change the CordovaWebView's onGeolocationPermissionsShowPrompt to xwalk.

XWalk adds the onGeolocationPermissionsShowPrompt support. Change the
CordovaWebView's implemenation to that.
Merge pull request #78 from huningxin/geolocation

Change the CordovaWebView's onGeolocationPermissionsShowPrompt to xwalk.
Bring back the CordovaWebView's WebBackForwardList related code.

XWalk adds the copyBackForwardList/saveState/restoreState API support,
and also adds WebBackForwardList and WebHistoryItem class.
Change the CordovaWebView's implemenation to that.
Merge pull request #79 from junmin-zhu/backforwardlist

Bring back the CordovaWebView's WebBackForwardList related code.
Merge pull request #76 from wang16/xwalk

Enable bin/create.js on Windows to create apps
Fix the unit test build error due to onGeolocationPermissionsShowPrompt support in XWalkCore

BUG=https://crosswalk-project.org/jira/browse/XWALK-357