ValueChecks: Alternative used syntax in ValueChecks

The main reason for starting this out was with the original intention of
migrating some util classes from the proprietary repo to this one, but
with the realization that there are quite some syntactic differences.

This commit is mostly for discussion, but also with a hope of aligning
the lingo in the two different util libraries so that a migration to
this one can be performed.

1. Removes the terms `defined` and `undefined`. This lingo feels awkward
to me. There is a term `null` in the language already: why complicate
things? Also `isNull` is shorter and clearer in my opinion.

2. Removes not-null and not-empty methods, as I think the class get
bloated with those.

3. Prefixes methods with a `is` to make it clearer that it's a question,
and that it doesn't perform any modification on the arguments. Also, for
example, `isEmpty` is standard in the java language.

4. Adds `ifNull` and `ifEmpty` methods to make code migration easier, as
those are present in the proprietary repo.
ValueChecks: Continuation of alternative syntax

* Using only language built-in null instead of undefined

* Still not using inverse functions, except for non-trivial functions.
  Added `any`, `all` and `none` for both null checks and emptiness

* Removed prefix `is`
Merge pull request #31 from iZettle/discussion-around-defined-undefined

[For discussion] ValueChecks: Alternative used syntax in ValueChecks