Add siblings-only finders (excluding the targeted node)

[chikamichi]

Hi,

This PR adds support for siblings-only finders and scope, eg. #siblings_only mainly.

Excluding the targeted node from the list of its siblings is as simple as node.siblings.reject { |s| s == node }, but it might make more sense to handle that within ancestry itself so as to leverage a cleaner, more efficient SQL query, and to provide a full-fledged API (eg. Model.siblings_only_of(node), Model#siblings_only_ids).

[coveralls]

Coverage increased (+0.05%) to 98.165% when pulling b851130 on chikamichi:feature/siblings-only into 68d6c05 on stefankroes:master.

[chikamichi]

Specs actually are passing, something's wrong on Travis for Ruby 1.9.3 when it comes to the config and some gems cannot be installed:

An error occurred while installing {foobar} (x.y.z), and Bundler cannot continue.

[kbrock]

@d-m-u wondering

• does it make sense to make siblings a configuration setting?
• is there an agreed upon standard for what you call all my parent's children?
• I think I like the simpler query and count > 1 vs complex query > 0

[kbrock]

related to #488

[kbrock]

I'm thinking this should be a breaking change and a config parameter would switch between the two?

thinking out loud and would love any feedback.