Short syntax for base assertions

Introducing new short syntax for base assertions:

```php
<?php

namespace tests\units;

use atoum;

class stdClass extends atoum
{
    public function testFoo()
    {
        $this
            ->variable('foo')->{'=='}('foo')
            ->variable('foo')->{'!='}('bar')

            ->object($this->newInstance)->{'=='}($this->newInstance)
            ->object($this->newInstance)->{'!='}(new \exception)
            ->object($this->newTestedInstance)->{'==='}($this->testedInstance)
            ->object($this->newTestedInstance)->{'!=='}($this->newTestedInstance)

            ->integer(42)->{42} // isEqualTo
            ->integer(42)->{'42'} // isEqualTo
            ->integer(rand(0, 10))->{'<'}(11)
            ->integer(rand(0, 10))->{'=<'}(10)
            ->integer(rand(0, 10))->{'>'}(-1)
            ->integer(rand(0, 10))->{'>='}(0)
        ;
    }
}

```

I was reviewing some code and remembered what we did with @mageekguy on mock call asserter to allow things like `$this->mock($foo)->call('bar')->{3}` to test if the `bar` method was called exactly three times on `$foo`. I asked myself if we could apply similar things to some other asserters, basically `variable` and `integer` to cover assertions that have a native operator counterpart (i.e `isEqualTo`, `isIdenticalTo`, ...).

As you have seen in the previous example, this is doable and works fine (I tested it from PHP 5.4 to 5.6). Let me know if you think it's useful ;)
```
switch (true)
{
   case is_numeric($method):
      $assertion = 'isEqualTo';
      break;

   case $method == '=<':
	$assertion = 'isLessThanOrEqualTo';
        break;

   ...
```