adding .gitignore
add Spectrum projection model (R and C++)

This adds the version of the model used for presentation to the UNAIDS Reference Group on 25 April 2014.

- R/: R implementation of the model
- C++/: C++ implementation of the model

Some notes:
- implements R-spline model (in R, based on Dan Hogan's code).
- vertical transmission and paediatric survival very crudely implemented
- migration not implemented
- ART apportioned only based on eligibility, not estimated mortality