Minor refactor in insert() and save()

[romaninsh]

This PR changes slightly how save() and insert() work.

With a more dynamic nature of the Agile Data creating data takes requires a lot of flexibility. There are three ways to add data currently:

• import([[data], [data], ..]); // returns $this
• insert([data]); // returns clone of $this
• save(); // save current record. Returns $this

The problems are:

• return of insert() is not very usable.
• save() requires to call set().

Proposal:

• make insert() return ID instead of clone. (often ID is good enough!)
• allow save([data]) which will combine set with save.