Escaped html inside text body if email is html

If there are html special characters in a text post, and that text post ends up in
an html email (i.e. because some other post in the email is html), then escape
those special characters with the htmlspecialchars function.
Added missing qa-app-posts.php source

Fixes #4

The function `qa_post_content_to_text` is used but the file in which
it's declared is not sourced. This is now fixed.
Added missing int cast for plaintext checkbox

Fixes #6

The checkbox for the option plugin_publish2email_plaintext_only was missing a
cast to int when fetching the field value in the POST. This has been fixed.
Merge pull request #3 from atavakoli/fix_html_in_plaintext

Escaped html inside text body if email is html
Merge pull request #5 from atavakoli/fix_source_app_posts_issue4

Added missing qa-app-posts.php source
Merge pull request #7 from atavakoli/fix_missing_checkbox_cast_issue6

Added missing int cast for plaintext checkbox
Added CHANGELOG.md

Up to date as of v1.1
Merge pull request #8 from atavakoli/master

Release 1.1