Add customdriver method to Persist module

[fohletex]

I recently ran into issues, where I wanted to use ForerunnerDB, but had no access to either window.Storage nor IndexedDB, WebSQL and localStorage. I further didn't want to use NodePersist. Hence, some research brought me to the localforage#defineDriver-API. Thus, I extended the Persist module with a customdriver method, which allows you to import a custom localforage driver for any other storage destinations, than the available ones.

[fohletex]

hmmm ... testbear fails by travis CI - on my machine it perfectly passes .... - seems as if there is a version conflict in the setup configuration - could that be?

                    _     _
                   ( \---/ )   
                    ( . . )
             ___,--.(__Y__),--.___
            |   `--'       `--'   |
            | Test Bear is HAPPY! |
            |    _,-.     ,-._    |
            |___(ooO )___( Ooo)___|
                 `--'     `--' 

EDIT: Saw, that the error is already documented: #246

[Irrelon]

If testbear is happy, I am happy :)

[Irrelon]

Actually that's some nice work there, thanks for documenting as well!

[Irrelon]

Merged!