Upgrade to CLD2

See #8.

Before this is merged, we should update our licensing. The library [has changed](http://src.chromium.org/viewvc/chrome/trunk/src/third_party/cld/LICENSE) to the [Apache license](https://code.google.com/p/cld2/source/browse/trunk/LICENSE).

The size of the bundled library has grown significantly. The source itself is over 90 MiB. The gem is now 35 MiB (up from 6 MiB) and installed it uses 93 MiB (up from 17 MiB). If CLD2 ever releases a tarball, we can stop bundling it here and shrink the installed size to 2 MiB.

There are two possible CLD2 libraries to link against: `libcld2.so` and `libcld2_full.so`. The latter can detect twice as many languages and is 4 MiB larger. I arbitrarily chose the former, smaller library in this PR. Which would you prefer to be used by default? In either case, we can also make this configurable during `gem install`.