Use `SQLTree#to_sql` instead of `arel.to_sql`

Because sometimes `arel.to_sql` requires bind values and
activerecord raises error if empty bind values.
Merge branch 'develop_2_x' into develop
Upgrade rspec to 3.1
Split support files
Use symbol because establish_connection's string argument is deprecated behavior
Use new hash syntax
Reduce logging to stdout
Remove unused logger replacement
Impl specs about reverse associations of clever loading
Remove unused files
Remove specs for rails 3.1
Fix deprecation warning
Remove logging output to stdout
rspec3 syntax
Remove unused code
Refactor finder spec
Fix clever_load
Update gemspec
Add spec for `Cluster#shard_for`
Fix Guardfile