v3.0.0 — code refactor, first basic tests

• Improved code structure: major functionality split out into separate files to support adding proper unit tests
• First very basic tests
• Added rate limiting honouring with a linear back-off strategy
• API server HTTP response headers are passed down to callbacks (inside the third argument to the callbacks, under response.headers)
• Added ability to find users based on name (pipedrive.Users.find({ term: 'John Smith' }, ...);)
• This version has no breaking changes compared to 2.x but given the scope of changes in the code and previous lack of tests, we decided to release it as a new major version. Thus, all 2.x users are strongly encouraged to migrate to 3.x.