Improving performance by eliminating unnecessary speller creation and deletion

Previously, an internal speller object was created and destroyed for every call to `#correct?` and `#suggestions`. Now, an internal speller object is only allocated when the initial `FFI::Aspell::Speller` object is created and when its configuration changes.

This does make the API more complex since users now need to call `#close` (or use `#open`) to free the internal speller object, but the performance gains are significant. Using my little [wordament-solver](https://github.com/schmich/wordament-solver) project, I get the following performance numbers:

- 1,207,100 calls to `#correct?`
- Before fix
  - `ruby -I../ffi-aspell/lib solver.rb  2118.34s user 320.91s system 98% cpu 41:24.01 total`
  - 570 calls/s to `#correct?`
- After fix
  - `ruby -I../ffi-aspell/lib solver.rb  7.08s user 0.45s system 99% cpu 7.590 total`
  - 170,494 calls/s to `#correct?`

In other words, the runtime of the solver dropped from about 41 minutes down to 7 seconds (~327x improvement) with this fix.

I've included updated documentation and README for `Speller.open`, `#close`, and `#closed?` as well as 7 new tests.

This is a significant enough change to warrant discussion. These changes create a compatibility issue in the public API since existing users will now be effectively leaking the internal speller object. I would suggest possibly bumping the major version number to indicate backwards-incompatible changes (see [SemVer](http://semver.org/)).

Let me know what you think. I'm open to feedback.