Tomographer

* Switched to [`pybind11`](https://github.com/pybind/pybind11) for Python
bindings (thanks to Chris Granade for suggesting this great library),
dropping the dependency on Boost.Python.

* Python package available with `pip`. Added module `tomographer.include` as
well as member `tomographer.version.compile_info` for packaging.

* Any custom figure of merit can be specified as a user-defined function to
the `tomographer.tomorun.tomorun()` in the Python interface

* OpenMP is no longer required neither for `tomorun` nor for the Python
interface. This allows to compile with Apple's default clang compiler on
Mac OS X. (OpenMP can still be used instead of C++11 threads, if
preferred.)

* New Python interface (using Boost.Python)

* Test integration with Travis CI

* Overhaul of some C++ components, some enhancements. Added for instance
`Tomographer::MHRWTasks::ValueHistogramTasks`

* Minor improvements to the main `tomorun` program

* Revised framework API for better organization, consistency and flexibility

* Changed a few default parameter values to more sensible values

* Improved error reporting for faulty inputs

* Fixed small bug where the "purified distance" figure of merit would always
use the MLE as reference state

* Small changes to documentation and build system

* First release of Tomographer.