Add --exclude_pattern option to configuration
Clarify wording.
Add spec clarifying behavior of `pattern` and specified files.

See https://github.com/rspec/rspec-core/pull/1651#discussion_r15448877 for background discussion.
Prefer `it "warns..."` to `it "will warn..."`.
Ensure --exclude-pattern still applies when `files_to_run` is accessed early.

This is based on 4cabc4143b4b99a8ea47fc702f33b60f86e3d4a1 from #1652.
Fix `exclude_pattern` so it takes affect after `files_to_run` is accessed.

Based on 6a5f3c60c419773625c094f27652ff5fc46af5af from #1652.
Add support to the rake task for exclude pattern.
Add changelog entry.
Merge pull request #1671 from rspec/pr-1651-rebased

Exclude pattern rebased