Python model files

This branch implements model file in Python. The model file, which defines the density, temperature, velocity and abundances are read by LIME at run time. Other input parameters are read from a separate input file. This has several advantages over the previous implementation:

1. The code does not need to be recompiled for each different model files
2. Model can be more easily written than in C
3. Other parameters are set from an input file rather than from a complicated C structure

In addition, the branch implements a CMake based build system. The code is built on a continuous built system (Travis CI) to make sure that the code compiles and run fine.