No reason to keep `method_missing` in its own file.
Move custom matcher base class into dsl.rb

This is the only place where it's used and it belongs here.
No need for a separate errors file.
Cleanup code org in built_in.rb
Move operator matchers into `built_in` directory.

Lazy loading it with autoloading is nice since
it won't be even be used when folks use only
the `expect` syntax.
Remove expression generator logic.

Given we are recommending the use of `expect`
in RSpec 3, no need to include `should` in
error messages.
Prefer `module_function` to `extend self`/`def self.`.
Move specs into files where they better belong.
Merge pull request #389 from rspec/some-general-cleanup

Some general cleanup