Add "Log Out" link to the settings dropbox menu.

This adds a logout link to the mobile site.

Before:

<img src="https://f.cloud.github.com/assets/1447206/1461885/633066c0-44d5-11e3-92ed-3c673ce121b2.png" width="384"/>

After:

<img src="https://f.cloud.github.com/assets/1447206/1461886/6332d266-44d5-11e3-93be-93e2059413d7.png" width="384"/>

The logout link in the normal settings menu is already hidden on line 2639 in _site.scss (since I meant to do this in the original commit):

    .settings {
	    .content {
		    display: table;
		    border-collapse: collapse;
		    width: 100%;
		    .settings-nav {
			    list-style-type: none;
			    display: table-cell;
			    width: 40%;
			    padding-top: 20px;
			    > * {
				    margin-right: 20px;
				    margin-left: auto;
			    }
			    @media (max-width: 527px) {
				    display: none;
			    }
			    .logout-link {
				    display: none; <--- here
			    }
		    }
		    /* etc.
	    }
	    /* etc. */
    }