Exclude exe/rspec from backtraces.

When users had installed rspec-core as a bundler :git dependency,
the rspec binary (locally defined in `exe/rspec`) would be included
in backtraces. This wouldn't affect users who had done a normal
gem install since rubygems copies it to a `bin` directory.

Fixes #1646.
Only ignore JRuby files on JRuby.

This should make things slightly faster on MRI as it’s
one less regex to check.
Remove unused file.
Update changelog.
Merge pull request #1648 from rspec/remove-exe-rspec-from-backtrace

Remove exe rspec from backtrace
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.