Add c client user guide.
Add Arcus C client user guide link in README file.
Revert to the original interface of arcus_connect() function.

Remove a pool argument from arcus_connect() function.
Remove memcached_last_error checking in memcached function testing.

In Naver eco-square VM environment that I used for testing,
both IPv6 and IPv4 addresses are enabled.
At first, the client uses IPv6 to connect to a cache node.
But, the connect request fails with "connection refused" error.
And, the next connect request with IPv4 get the success response.

According to IP settings of test machine,
memcached_last_error might be set with some errors.
To make "make test" to succeed in any environment,
we remove memcached_last_error checking.
Fix a compile error: missing semi-colon(;) at the end of function declaration.
Update arcus c client behavior description.
Update 02-arcus-c-client.md
Update 02-arcus-c-client.md
Modify the memcached.h file to include the arcus.h file.
Disable registering client info in ZK ensemble.
Merge branch 'master' into replication