WebSetting plugin implementation

The Web setting extension defines APIs that manages the setting states of the Web view in Web application.
Properties of the Web view that can be managed via Web setting API:
- Delete all cookies saved for the web view in the Web application
- Set a custom user agent string of the web view in Web application

This commit includes implementation of API for Web application to set custom user agent and
remove all application cookies.

Result of running tct-tizen-websetting tests: 22/22 pass.

BUG=XWALK-1063
Merge pull request #371 from jizydorczyk/master

WebSetting plugin implementation