Remove parsing, selector logic from the trait; introduce custom constraints

[stevegrunwell]

In preparation for version 2.x of this library, this PR introduces a series of custom PHPUnit constraints (based on PHPUnit\Framework\Constraint\Constraint), enabling greater flexibility with regards to how assertions are handled.

For example, assertContainsSelector() currently relies on counting the number of matches:

$this->assertGreaterThan(0, count($results), $message);

When the assertion fails, users see a message like "Failed asserting that 0 is greater than 0", which isn't particularly helpful.

With the new version, we can get messages like this:

Failed asserting that '

This is unexpected

' contains selector '.outer .expected'.

The new constraints also make code much easier to test and re-use across assertions.

The Selector class

Rather than having a series of internal methods end up exposed on implementing test classes, this PR uses a new Selector class, which acts as a value object for CSS selectors. If given an array of attributes, this is automatically converted into the appropriate CSS selector.

The DOM class

This PR also defines the DOM class, which is a simple wrapper around Symfony's Crawler class that exposes the methods necessary to query using Selector objects.

Backwards compatibility

In order to be compatible with current versions of PHPUnit, this PR does raise the minimum PHP version to 7.0. While I want to keep support as broad as possible (after all, this originally started because I had to try to write some regression tests around WordPress), the fact that version 1.x is still available for those poor, unlucky souls means they aren't totally left out in the cold.

[coveralls]

coverage: 100.0%. remained the same
when pulling f85fca4 on refactor/remove-logic-from-trait
into 62cfbb1 on develop.