Add more semantic constructors

[haampie]

This PR adds two methods which allow a more intuitive way to construct a parser, e.g.:

$parser = Parser::fromUserAgent($request->header('User-Agent'));
$parser = Parser::fromHeaders($request->headers());

As a side note: two redundant instanceof tests are removed. An instance either fails to construct or is already an instance of that class.

[coveralls]

Coverage remained the same at 100.0% when pulling e60a1e7 on haampie:feature-static-constructors into fcec4e9 on WhichBrowser:master.

[ThaDafinser]

Different ways to achieve the same result are generally not good.
Also static calls should not be used to return instances of something

Also there is the analyse() method with the same signature.
https://github.com/haampie/Parser/blob/feature-static-constructors/src/Parser.php#L55

[haampie]

Different ways to achieve the same result are generally not good.
Also static calls should not be used to return instances of something

Named constructors are actually quite common (e.g. here or here). What is wrong with the concept in your opinion?

Also there is the analyse() method with the same signature.

Maybe this is a matter of taste, but I would rather not have such a method; it makes the state of the object mutable / not fully encapsulated. The method could be private for instance.

But this method is there probably because the constructor is expensive as well? In that case, must the parser already be constructed with a user agent string? Is the user agent string actually part of the state of a parser?