Refactor specs in prep for rake task pattern changes.

This detects the loaded files by parsing the command
produced by the rake task, and running it through
the option parser and configuration.
use rspec --pattern option in rake tasks

rspec has an internal option (--pattern) to receive pattern of files to run
via the command line, use this option in rake_tasks instead of passing through
a huge list of files one by one on the command line.

Fixes #1126.
Remove unnecessary -S from rake task.

Fixes #1559.
Add changelog.
Make `script/rspec_with_simlecov` act like `rspec`.

In RSpec::Core::Configuration#files_or_directories_to_run=
we check to see if `$0` is "rspec", so we need to fake
it out here.
Rename method.

It no longer returns a list of files to run, but rather returns a
file specification in the form of either a `--pattern` option or a
list of files.
Merge pull request #1653 from rspec/1589-fixed-up

Pass `--pattern` from rake task (#1589 fixed up)