Make improvement to isNeeded method

[happyhj]

Issue

382

387

Details

• make improvement to isNeeded method
  • made research for browser coverage of bfcache (https://github.com/naver/egjs/issues/372#issuecomment-261003875)
  • add dependency to eg.agent
  • add lots of unit tests for isNeeded
• update isNeeded jsdoc
  • update isNeeded method jsdoc
  • add persist custom event jsdoc

Preferred reviewers

@netil @jongmoon

[netil]

Also, I want point out for the duplicated items showing on API doc.

It happens because of these comments:

/**
    * Stores the current state of the web page in a default key using JSON.
    * @method jQuery.persist

/**
    * Return stored state in global namespace as object
    * @method jQuery.persist

/**
    * Stores the current state of the web page using JSON.
    * @method jQuery.persist

Those can be described as one using JSDoc's optional param syntax.

/**
    * Get or store current state of the page context
    * @method jQuery.persist
        * @param {String} [key] The key of the state information to be stored
        * @param {Object} [state] The value to be stored in a given key
        * @example
        *    // example1 when only 'key' is given
        * @example
        *    // example2 when 'key' and 'value' are given
         ...