Normalize text properties of a tweet with NFC.

* twittering-mode.el: Normalize text properties of a tweet with
the Normalization Form Canonical Composition (NFC) defined in
Unicode Standard Annex #15.
(twittering-normalize-string): new function.
(twittering-make-gap-list): takes account of the normalization of
a tweet text with NFC.
(twittering-extract-common-element-from-json): normalize a tweet
text with NFC.
(twittering-json-object-to-a-status-base): normalize a source,
name, location and description of a tweet with NFC.