Authorizes transactions

Removes "require_admin_sign_in" filter from Transactions Controller and utilizes an authorize_transaction helper method instead that looks for log-in credentials (i.e.) email and password in the params string. It will only add new transaction if it is able to authenticate the email/password combo. 

I had some concern over sending email/password combo in the post request. However, from what I understand in my research into the issue, as long as they use https it should be alright with a post request.  

http://security.stackexchange.com/questions/33793/handling-passwords-in-a-web-application

@melissa-mccoy 
i guess, as long as it needed to happen anyways, it's good. i'll merge now!