Unify method naming

Current httpexpect API suffers from a problem: it's not always clear what a method returns: the same object on which it was called, or a new object.

To address this problem, two steps will be done:

* method naming will be unified, so that it's easy to guess from the name what is returned
* a new set of methods will be added for inspecting nested objects using callbacks, so that you can write a bit more verbose but also more structured assertions, if you want

This issue is for the first part; the second will be tracked in a separate issue.

Renaming methods will be a gradual process. A few subsequent releases will add methods with new names and mark old methods as deprecated. All deprecated methods will be kept until release of v3. There will be a time window between deprecations and release of v3 so that the users have time to update.

The following naming convention is suggested:

* assertions - `IsXxx()`, `InXxx()`, `HasXxx()`, `ContainsXxx()`, `ConsistsXxx()`

Methods that perform a check on the object and return the same object, will all have distinguishable prefixes listed above. Examples: `IsEmpty`, `InRange`, `HasSuffix`, `ContainsAny`. Many of these methods are already named this way.

* converters - `AsXxx()`

Methods that convert given object to another type and return it, will have prefix `As`. Examples: `AsNumber`, `AsDateTime`. All such methods are already named this way.

* accessors - `Noun()`

Methods that extract another (new) object from the given one and return it, will keep their names and will be named just as nouns. Examples: `Body`, `Form`, `Element`.

I was thinking about given them a prefix as well (e.g. `GetXxx`), but this would break too much code and significantly harm brevity.

Actual renames to be done:

- [x] All: `Empty` => `IsEmpty` (**2.11.0**)
- [x] All: `Equal*` => `IsEqual*` (**2.11.0**)
- [x] Boolean: `True`, `False` => `IsXxx` (**2.12.0**)
- [x] Value: `Null` => `IsNull` (**2.12.0**)
- [x] Array: `Element` => `Value` (**2.13.0**)
- [x] Object: `ValueEqual` => `HasValue` (**2.13.0**)
- [x] Response: `ContentType`, `ContentEncoding`, `TransferEncoding` => `HasXxx` (**2.16.0**)
- [x] Match: `Index` => `Submatch` (**2.17.0**)
- [x] Match: `Name` => `NamedSubmatch` (**2.17.0**)
- [x] Match: `Values` => `HasSubmatches` (**2.17.0**)
- [x] Cookie: `HasMaxAge` => `ContainsMaxAge` (**2.17.0**)
- [x] All: `Gt`, `Ge`, `Lt`, `Le` => `IsXx` (**2.18.0**)
- [ ] Response: `Status` => `HasStatus` (**2.18.0**)
- [ ] Response: `StatusRange`, `StatusList` => `HasStatusInXxx` (**2.18.0**)
- [ ] WebsocketMessage: `Is`, `HasType`, `HasCode`