Updated sync URL, and cleanup

* Updated default sync URL to demo.mobile.couchbase.com
* Don't crash in setup code if Couchbase Lite can't be initialized
* Use -[CBLDocument update:] to change docs
Simplified and cleaned up the code.

* Removed configurable sync URL; it adds too much complexity.
* Adopted current best practices with CBL API, i.e. using -[CBLDocument update:].
* Modernized code, e.g. moving ivar declarations to .m file.
* Added "#pragma mark" markers to easily find interesting code.