Rework documentation

- adjust to line break at 100 characters
Add more tests and use Google's test framework
Add/change documentation
Clean up .gitignore and configure.ac
Change interface of mc_point

- `point` is now declared const
- updated `vegas_point` accordingly
- use division instead of division and multiply in `vegas_adjust_grid`
Renamed linear_grid to piecewise_constant_pdf

- moved code from VEGAS into the new class
- add test for the function `icdf` and use `EXPECT` instead of `ASSERT`
Use `EXPECT` instead of `ASSERT` in tests