Cleanup NET45 build. Add missing NUnit lib.
Fix #303

* Add SafeAddToChangesCount() method which uses InterLocked to increment the changesCount value.

* Modify Puller and Pusher to use the SafeAddToChangesCount() method.
Merge pull request #305 from couchbaselabs/issue-303

Fix #303 ChangesCount Error
Fix #304 Too many open files

As the HttpClient's handler is not shared anymore, we need to ensure that the HttpClient is disposed after done using it.
Merge pull request #306 from couchbaselabs/issue-304

Fix #304 Too many open files
Merge branch 'master' of https://github.com/couchbaselabs/couchbase-lite-net into 20140921_master
Made id's lowercase.