Change Sort to have two static constructors

[Alexander Makarov]

After #66 Sort is less usable for non-user input case. You have to specify all fields all the time and it's tedious.

I propose to have two static constructors:

• Sort::fields() for user input with specifying allowed fields.
• Sort::any() for non-user input allowing any fields.

[Aleksei Gagarin]

Sort::fields(), Sort::any()

mb it will be more consistant:
Sort::only(), Sort::any()
or Sort::external()/Sort::internal() with the appropriate method isExternal()

[Alexander Makarov]

only/any sounds alright.