Switch to composer

- adds a `composer.json`
- removes Raven submodule as it's dependency is expressed via composer now

Fixes Sandreu/cake-sentry#9

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- the composer package name is `sentry`, because only then it gets installed as `Plugin/Sentry`; otherwise it would end up as `Plugin/CakeSentry` ... haven't found a way to retain the github repo name and change the destination folder
- I choose the `BSD 3-clause` license because it contains:<br><br>*Neither the name of the copyright holder nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.* which I think is a nice thing<br><br>Otherwise e.g. MIT would be as good
- `email` needs to be filled in `composer.json` (not sure if it can be left out?)
- I removed the git submodule due composer dependency resolution; that also required adapting the sources and removing the `App::import()` statements; i.e. raven is automatically installed as part of a cake sentry installation via composer and since CakePHP2 is using composer it automatically finds Raven. I can confirm this because this is how we're using it in production