[Debug] added the component (closes #6828, closes #6834, closes #7330)
added a main Debug class to ease integration
[Debug] made the exception handler independant of HttpFoundation
[Debug] ensured that the Debug tools can only be registered once
[Debug] changed composer to accept more versions
[Debug] fixed minor bugs
fixed typos
merged branch fabpot/debug-component (PR #7441)

This PR was merged into the master branch.

Discussion
----------

[Debug] added the component (closes #6828, closes #6834, closes #7330)

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #6828, #6834, #7330
| License       | MIT
| Doc PR        | symfony/symfony-docs#2479

You can use the individual tools, or register them all:

```php
use Symfony\Component\Debug\Debug;

Debug::enable();
```

Changes in Symfony SE: symfony/symfony-standard#523

Commits
-------

f693128 fixed typos
1ab1146 [Debug] fixed minor bugs
daa3a3c [Debug] changed composer to accept more versions
e455269 [Debug] ensured that the Debug tools can only be registered once
946bfb2 [Debug] made the exception handler independant of HttpFoundation
2b305c2 added a main Debug class to ease integration
2ff0927 [Debug] added the component (closes #6828, closes #6834, closes #7330)