Initial develop branch commit
Improve user interface text

* Marks up the help text as description, and moves it to below the settings.
* Simplifies the labels.
* Gives less emphasis to Mailchimp, since the same plugin could be used to make the same amendments and let another service do the actual sending of emails from an RSS feed.
Internationalize strings

Uses text domain which matches the plugin slug (send-images-rss).
Refactor all the things

* Add prefixes to classnames.
* Update file names to match
* Move main plugin class into new file.
* Remove instantiation calls out of class files, so that requiring the files has no side effect.
* Make the feature classes dependencies of the main class, so the main class can hook in the feature classes where necessary.
* Remove constructors from feature classes, as they are no longer called.
* Switch away from "alternate feed class" approach to just registered a new feed, and conditionally choosing whether to apply the fixes or not.
* Updated the UI to be more consistent with WP defaults. Use of description HTML class, descriptions moved under input and simpler labels. Removal of prominent MailChimp references, since the functionality works even if another service is accessing the feed.
* Internationalization of strings. Would still need the text domain loading, and a .pot file included in the plugin.
* Add documentation throughout. Left @since tags as x.y.z so they can be filled in accurately later on.

BACKWARDS COMPATIBILITY: Renamed some fields to be more logical. Some Mailchimp-referenced values could also be updated. Or just ignore these changes if you don't want to deal with BC right now.

Needs to be tested more fully (this project looks to be a good candidate for writing some unit tests, now things are structured a little better) before consideration is giving to merging to master.
Merge pull request #1 from GaryJones/develop

Updates from Gary
the_content, not the_content_feed
admin settings updated
warnings if multiple options selected
implemented actual warnings.
get the correct option
corrected gallery_stripper to run on the_content_feed
default size for sendimagesrss_image_size (issue #6)
esc_attr later (issue #8)
textdomain, pot (issue #4, issue #5)
load_textdomain()
updated .pot
strip() moved to the_content again, inside is_feed. Running it on the_content_feed overrode fix()
corrected sanitization, thanks Gary (issue #7)
moved descriptions to help tab (issue #2)
moved descriptions to help tab (issue #2)