Add ConsistOnlyOf verification extension methods for IEnumerable<T>

[YevgeniyShunevych]

• Verifies that collection consists only of items that match the predicateExpression.
• Verifies that collection consists only of items that equal to the expected value.

Usage

new[] { 1, 2, 3 }.ToSubject()
    .Should.ConsistOnlyOf(x => x > 0);

new[] { 3, 3, 3 }.ToSubject()
    .Should.ConsistOnlyOf(3);