[Security] Added Security\Csrf sub-component with better token generation

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | TODO

**Update September 27, 2013**

This PR simplifies the CSRF mechanism to generate completely random tokens. A random token is generated once per ~~intention~~ token ID and then stored in the session. Tokens are valid until the session expires.

Since the CSRF token generator depends on `StringUtils` and `SecureRandom` from Security\Core, and since Security\Http currently depends on the Form component for token generation, I decided to add a new Security\Csrf sub-component that contains the improved CSRF token generator. Consequences:

* Security\Http now depends on Security\Csrf instead of Form
* Form now optionally depends on Security\Csrf
* The configuration for the "security.secure_random" service and the "security.csrf.*" services was moved to FrameworkBundle to guarantee BC

In the new Security\Csrf sub-component, I tried to improve the naming where I could do so without breaking BC:

* CSRF "providers" are now called "token generators"
* CSRF "intentions" are now called "token IDs", because that's really what they are

##### TODO

- [ ] The documentation needs to be checked for references to the configuration of the application secret. Remarks that the secret is used for CSRF protection need to be removed.
- [ ] Add aliases "csrf_token_generator" and "csrf_token_id" for "csrf_provider" and "intention" in the SecurityBundle configuration
- [x] Make sure `SecureRandom` never blocks for `CsrfTokenGenerator`