Migrate libcouchbase to couchbase-ruby-client

[fab-girard]

Update README:

Stat	Couchbase Ruby Model	Couchbase-ORM
Transactions	2500 hits	2500 hits
Elapsed time	12.24 secs	6.82 secs
Response time	0.88 secs	0.34 secs
Transaction rate	204.25 trans/sec	366.57 trans/sec
Request Code	ruby-model-app	couch-orm-app

[fab-girard]

check the behavior when gem is loaded/initialized with unavailable couchbase

[fab-girard]

#33
#40