'IF NOT EXISTS' for insert statement

The usage is just like 'ttl'.
rename all 'check_exist' related things to 'if_not_exists'
Merge remote-tracking branch 'origin/16' into if_not_exist_insert
'IF NOT EXISTS' for insert statement

The usage is just like 'ttl'.
rename all 'check_exist' related things to 'if_not_exists'
Merge branch 'if_not_exist_insert' of https://github.com/mission-liao/cqlengine into if_not_exist_insert
rename keyspace for testing
update docs
Merge branch 'master' into if_not_exist_insert
Merge branch 'master' into if_not_exist_insert
if not exist only supported by cassandra 2.0 or later.
if not exist is a new feature of cassandra 2.0
removing parameter of if_not_exists in QuerySet
refine code with skipUnless
add test case for batch query
refine doc
'IF NOT EXISTS' for insert statement

The usage is just like 'ttl'.
rename all 'check_exist' related things to 'if_not_exists'
'IF NOT EXISTS' for insert statement

The usage is just like 'ttl'.
rename all 'check_exist' related things to 'if_not_exists'