Simplify New Tweet function now that t.co link shortening is built
into 
  - Removed is.gd shortening service
  - Refactor methods for determining number of characters remaining
  - TODO: Dynamically fetch max link sizes from configuration API
Changes to Twitter dashlets config
 - Increase timeline dashlet polling period to 300s (5 mins) to take
account of lower limits for authenticated requests in v1.1 API.
Period is now the same for all three dashlets (since all are
authenticated)
 - Load 50 tweets at a time rather than 20 to minimise number of API
requests
Dymanically load max URL sizes for t.co shortened links (now always on)
and use this to compute tweet char counts
Use tweet entities data to better display t.co-wrapped links embedded
within tweets