Initial commit that introduces UDF (user defined flags).

Memcached.txt specifies that flags are a 16-bit integer. Since the library uses
internally 8 bits to store type, compression and serialization, this leaves us
8 bytes left to store user data. So basically added the udf_flags parameter to
all relevant functions to store and retrieve them.
Was a bit too greedy with all the errors
updated memcached documentation
Final fixes and tests, cleanup
Merge remote-tracking branch 'parent/master' into feature-udf
Fixed broken test tests/cas.phpt.

Now, $cas_tokens doesn't need to be passed by reference in the get, getMulti, getByKey, getMultiByKey.
Both by reference and by value are supported, but cas will only be enabled if it's passed by reference.
It's handy if you want to use the udf_flags but you don't want to enable cas.
Merge branch 'master' of github.com:php-memcached-dev/php-memcached into feature-udf
Merge branch 'feature-udf'