Move the issue body into the table view as a new section.
Mess with the constraints a bit.
## Increased Elegance
* GHIssue and GHComment are now unified by the LETalk protocol
* LETalk implementation is available as a category
* GHIssue and GHComment simply import GHManagedObject+LETalk instead of GHManagedObject.h and the rest is magic
  * This relies in a non-fragile, but non-nil-safe way on some properties GHIssue happen to share with GHComment

## LETalkCell improvements
* Configures itself given an LETalk
* Shows a styled user name and a time stamp

## GHUser improvements
* Add known, but currently undelivered property names for GHUser
  * These can be filled in by loading the user object directly on insert / fetch
  * Which in turn can lead to a nice encapsulated loading chain
  * And which might also solve our updating / duplicates problem

## Other changes
* Disallowed editing in LETalkViewController
* Add macros for emptiness and nil replacement to the pre-compiled header
* Expose properties on model objects as necessary
Merge remote-tracking branch 'origin/master'