allow user to insert gallery shortcode with options

git-svn-id: http://gsoc.svn.wordpress.org/2013/codebykat/post-by-email/trunk@2359 f69abe3c-2ee2-4550-9a6d-17bd458115be
Added warning when checking email before saving changes to settings

git-svn-id: http://gsoc.svn.wordpress.org/2013/codebykat/post-by-email/trunk@2360 f69abe3c-2ee2-4550-9a6d-17bd458115be
added screenshots, contextual help and expanded Readme

git-svn-id: http://gsoc.svn.wordpress.org/2013/codebykat/post-by-email/trunk@2361 f69abe3c-2ee2-4550-9a6d-17bd458115be