Basecampapi

- Enabled multiline string on Campfire.write()

- Added upload_from_bytes() method to Attachments()

- Removed necessity to define sessions in each object, credentials are now automatically passed to each object after authentication is established through Basecamp object
- Removed the Authorization object and merged its functionality with Basecamp object
- Account_id is now defined inside credentials dictionary
- New object Attachments now replaces Basecamp.upload_file() method
- Improved initial authentication flow

- Enabled all python versions 3.7 and higher
- Fixed dependancy errors
- Upgraded ease of use for file uploads

- Fixed a bug with dependencies not being imported

- Created Authorization object for easier initial authentication
- Easier package import - now it's enough to only `import basecampapi`
- Added exception raising for bad requests
- Added docstrings to all objects and their methods
- Full documentation on README.md