support for uploading a file to thingiverse form a string

current upload method requires a File object to upload to Thingiverse. This can be problematic in some hosting environments, where temporarily files cannot be created.

This changes does two things:
- added ability to upload from a string
- optionally specify the intended thingiverse filename for the uploaded file

From the source:
---
    # file_or_string can be a File or a String.
    # thingiverse_filename is optional if using a File (the File filename will be used by default) but is required if using a String
    def upload(file_or_string, thingiverse_filename=nil)
---

added tests to test/unit/api_test.rb:
- test_file_upload_with_file_object_default_name (previously named test_file_upload)
- test_file_upload_with_file_object_explicit_name
- test_file_upload_with_string_object
cleanup test_helper.rb

- change to public URLs
- set id/secret/token to empty
Merge pull request #4 from sunekaae/feature/upload-from-string

Feature/upload from string