Fix posting issue with Emoji, but in general with illegal character that are outside the XML 1.0 Charset

Fix for #809 

When one or more *Illegal Characters is found, and the app can't serialize the content, a recovery/clean procedure is called. In details the app does the following steps when serialize fails:
1. Checks and replaces Unicode surrogates characters with HTML Entity.  (While replacing checks if the character is an emoji, and tries to use textual version of it if available on WP).
2. Tries to serialize the resulting string.
3. If it fails again, strips characters that are not allowed in XML 1.0 and serializes again.

*Illegal Character = Characters outside the XML unicode charset as specified by the XML 1.0 standard. http://www.w3.org/TR/2000/REC-xml-20001006#NT-Char