added email templates for selling listing

bookshare/trades/email/sold.html

+<p>Hey there, <strong>{{ bidder_first_name }}</strong>!</p>
+
+<p>Great news!</p>
+
+<p>Seller <strong>{{ seller_name }}</strong> has picked your bid for
+<strong><em>{{ listing_title }}</em></strong> on
+<a href="http://bookshare.srct.gmu.edu/">SRCT Bookshare</a>.
+They're the cc'ed email address-- <strong>{{ seller_email }}</strong>. Watch
+your email to arrange all the final touches to get your book.</p>
+
+<p>{{ email_message }}</p>
+
+<p>Thanks for using SRCT Bookshare! :-)</p>
+
+<em>(Heads up: no one watches this email address. If you need to get in touch
+with us, for any reason, see srct.gmu.edu/contact.)</em>
+
+---
+
+<small><a href="http://srct.gmu.edu/">SRCT</a> is a
+<a href="https://si.gmu.edu/registered-student-organizations/">
+registered student organization</a> at
+<a href="http://www.gmu.edu/">George Mason University</a>. We have written and
+provide this project for the benefit of our school's community, but offer no
+claims to the accuracy of the information posted on our site by other students.
+The source code for this project is available and
+<a href="https://www.gnu.org/licenses/gpl-3.0.html/">freely licensed</a> at 
+<a href="http://git.gmu.edu/srct/bookshare/">git.gmu.edu/srct/bookshare</a>.</small>

bookshare/trades/email/sold.txt

+Hey there, {{ bidder_first_name }}!
+
+Great news!
+
+Seller {{ seller_name }} has picked your bid for {{ listing_title }} on SRCT
+Bookshare. They're the cc'ed email address-- {{ seller_email }}. Watch your
+email to arrange all the final touches to get your book.
+
+{{ email_message }}
+
+Thanks for using SRCT Bookshare! :-)
+
+(Heads up: no one watches this email address. If you need to get in touch with
+us, for any reason, see srct.gmu.edu/contact.)
+
+---
+
+SRCT is a registered student organization at George Mason University. We have
+written and provide this project for the benefit of our school's community, but
+offer no claims to the accuracy of the information posted on our site by other
+students. The source code for this project is available and freely licensed at
+git.gmu.edu/srct/bookshare.