Add missing oauthconsumer source files to make OAuth replication work

Without these you’d get exceptions at runtime.
Also updated the oauthconsumer library to prefix system class category
method names with “oa_”.
Fixes #210
API: Rename CBLQuery.updateIndex to .indexUpdateMode

Renamed the enum correspondingly.
For couchbaselabs/couchbase-lite-api#25
Added clarification to kCBLDocumentChangeNotification doc-comment

…to say that only existing instances post notifications.
Set ‘Skip Install’ build setting on CBL iOS and CBL Listener iOS targets

Fixes #204