Static factories for schema builders.

[Sergej-Popov]

As per the suggestion in #8 PR review.

Create static builder factories for creating schema objects.
Such fluent builders will be more convenient to use and easier to discover the API with IntelliSense.
For example:

const schema = new Schema<Model>()
        .with(m => m.StringProp, new AnyOf([
          "specificValueNotMatchingRegex",
          /^[A-z]+\.[A-z]+$/,
          new StringSchema({
            enum: ["123"]
          })
        ]))
        .build();

could be expressed as something like

      const schema = new Schema<Model>()
        .with(m => m.StringProp, jsonschema.matching.anyOf([
          "specificValueNotMatchingRegex",
          /^[A-z]+\.[A-z]+$/,
          jsonschema.string.exactMatch("123")
        ]))
        .build();