Added functionality to change e-mail account password

Added method  to change the password of an e-mail account using . 

http://docs.cpanel.net/twiki/bin/view/ApiDocs/Api2/ApiEmail#Email%3a%3apasswdpop

Email::passwdpop
API Version:	2 — Read the documentation for this version.
Description:	This function changes an email account's password.
Parameters:	domain (string)
 	The domain for the email address for which you wish to change the password. (For example, example.com if the address is user@example.com.)
 	email (string)
 	The username for the email address for which you wish to change the password. (For example, user if the address is user@example.com.)
 	password (string)
 	The desired password for the account.
Returns:	
<data>
  <reason> A string value that is undefined if the password is changed successfully, or contains a reason for failure.</reason> 
  <result> A boolean value that indicates success or failure. A value of 1 indicates success. A value of 0 indicates failure.</result>
</data> 
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Thanks for your PR @sanjithckin. Can you add a blank line before this one? Also, can you adjust the spacing and line lengths in this comment section to be more consistent with the comments for the other methods above?