Omnisearch: Allow filters on get_posts.

WPML filters the get_posts call and changes it to return only posts in
the current language. By default, get_posts does not apply filters. If
we add the attribute, 'suppress_filters'=>false, we allow that filter
in WPML to work normally.

Props: WPML.org
Add a `wpml-config.xml` file.

This lets WPML recognize which of our options are strings that may need
to be translated.

A share feature option is a WP option, so to make it translatable we
need to add it to wpml-config.xml file. When plugins use get_option,
they read values from the wp_options table, WPML can filter these calls
and provide translation to the values of these options.

A greeting text option is a WP option, so to make it translatable we
need to add it to wpml-config.xml file. When plugins use get_option,
they read values from the wp_options table, WPML can filter these calls
and provide translation to the values of these options.

Props: WPML.org
Top Posts: Let WPML filter the results to the current language.

Add a new filter, `jetpack_widget_get_top_posts` that lets third-party
plugins filter the results of the top posts widget to specifically
exclude certain posts if desired.

Also adds in compatibility code (compliments of the fine folks at
WPML.org) that removes posts that aren’t in the same language as the
current ui.
Switch to strict comparison.
Pull the ifcheck wrapper around everything.

Fix some whitespace and indentation.
Let WPML translate contact form additional fields.

Contact form additional fields can't be translated because they are
created on the fly. WPML can register dynamic strings to translation
with icl_translate function.

So let’s let WPML do it! :)

Props WPML.org
Merge pull request #302 from Automattic/wpml-additions

Additions for better WPML compatability