Refactor and add test framework

[eywalker]

Performed refactoring of codebase for simpler import hierarchry and add basic testing framework based on pytest

• Use absolute import throughout the package. There appears to be no real concensus as to whether realtive import should be used or not (some interesting remarks here https://bugs.python.org/issue10031). As the structure of the package has become more complex, relative imports were getting quite counter intuitive, so I'm going to try making absolute imports throughout the package. Only exception to this is in __init__.py, where it is standard to do relative import.
• Reorganized modules to minimize potential circular imports
• Add implementation of Merge mapper as well as MergedQuerySource

[eywalker]

Superseded by #14