Feature/upload from string

Tony, I refactored the previous copy/paste for upload from string (instead of File). Ended up handling in 1 method, with some conditional logic based on whether the the first object is "File" or "String". Additionally, there's a second optional argument to specify the thingiverse filename (required if using "String").

Also added tests.

And cleaned up the token/id/secret/url stuff in the helper.

Shout out if you have any suggestions for improvements or refactors.

Btw., thanks to Robert Carlsen for feedback on the implementation.

/Sune