Added XBMC collection (set) categorization capability

The XBMC NFO generated by CouchPotatoServer lacked support for movie sets (collections): http://wiki.xbmc.org/index.php?title=NFO_files/movies#Movie_sets

CouchPotatoServer already scrapes data from TMDb, which provides collections information through with the tmdb3.tmdb_api.Movie object, and this change merely passes those that data into the NFO (XML) file that XBMC uses.

This pull request addresses this request: https://couchpota.to/forum/viewtopic.php?f=7&t=2496&sid=cb7df49a0c5f4479caedfe1e31cd3bfb

```diff
--- original.xml        2013-11-13 22:42:56.156021315 -0800
+++ new.xml     2013-11-13 22:42:47.152021352 -0800
@@ -1,30 +1,32 @@
 <?xml version="1.0" ?>
 <movie>
   <title>Star Wars: Episode I - The Phantom Menace</title>
   <id>tt0120915</id>
   <runtime>136 min</runtime>
   <year>1999</year>
   <plot>(32 Years Before Episode IV) Stranded on the desert planet Tatooine after rescuing young Queen Amidala from the impending invasion of Naboo, Jedi apprentice Obi-Wan Kenobi and his Jedi Master discover nine-year-old Anakin Skywalker, a young slave unusually strong in the Force.</plot>
   <tagline>Every generation has a legend. Every journey has a first step. Every saga has a beginning.</tagline>
   <rating>6.5</rating>
   <votes>357679</votes>
   <genre>Action</genre>
   <genre>Adventure</genre>
   <genre>Science Fiction</genre>
   <genre>Fantasy</genre>
   <genre>Sci-Fi</genre>
   <actor>
     <name>Ewan McGregor</name>
   </actor>
   <actor>
     <name>Liam Neeson</name>
   </actor>
   <actor>
     <name>Natalie Portman</name>
   </actor>
   <actor>
     <name>Jake Lloyd</name>
   </actor>
   <director>George Lucas</director>
   <credits>George Lucas</credits>
+  <set>Star Wars Collection</set>
+  <sorttitle>Star Wars Collection 1999</sorttitle>
 </movie>
```