Change CordovaWebView to derive from XWalkView.

The plain Cordova app is able to run.

TODO:
1. Plugin
2. Unit testing
Enable remote debugging.
Use XWalkSettings
Add Looper to the threat that plugin runs in.

This fixes the issue that Geolocation plugin does not work.
In XWalk, the thread which ExposedJsApi.exec() runs in does not contain
a message loop. As android.location.LocationManager need a loop to
process message, we have to prepare a Looper manually.
Fix file transfer plugin crash issue.

The root cause is the File Transfer API implementation leverages the android.webkit.CookieManager.
But trying to getinstance() of CookieManager before the webview isn't instantiated would cause crash.
In the cordova with xwalk backend solution, there doesn't exist webview.

According to android official document:
http://developer.android.com/reference/android/webkit/CookieManager.html#getInstance%28%29
We need to call "CookieSyncManager.createInstance(Context)" first.

Known issue:
This only fix crash issue, the cookie support is faking.
Still need to bridge xwalk cookie manager with default android.webkit.CookieManager.

BUG=https://github.com/otcshare/cordova-xwalk-android/issues/13
Add more detail comments and TODO for file transfer plugin crash issue.
Hook activity life-cycle events to XWalkView handlers.

It needs to hook activity life-cycle events (resume, pause, destory and
on activityresult) to XWalkView handlers. Otherwise, it causes black screen
when pause-resume cycles for apps.

BUG=https://github.com/otcshare/cordova-xwalk-android/issues/55
Enable the pause and resume timers based on XWalkView.

Enable them since XWalkView has the APIs.
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.
Add xwalk_core_library to git ignore path

Otherwise, buildbot will not pack it into tarball.
Add empty implementation for loadDataForTest in CordovaWebView

Otherwise, build is blocked for abstract function not implemented.
Implement the destroyExtension interface

It is introduced in XWalkRuntimeViewProvider to destroy the native
resources occupied by extension.
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.
Change the CordovaWebView's onGeolocationPermissionsShowPrompt to xwalk.

XWalk adds the onGeolocationPermissionsShowPrompt support. Change the
CordovaWebView's implemenation to that.
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.
Fix the unit test build error due to onGeolocationPermissionsShowPrompt support in XWalkCore

BUG=https://crosswalk-project.org/jira/browse/XWALK-357
Update the README.md

It includes:
1. update the project wiki URL.
2. add the python 2.6 into the requires.
Not to enable remote debugging by default.

BUG=https://crosswalk-project.org/jira/browse/XWALK-457
Update the README.md according to the wiki link update.