added travis ci file
Merge pull request #24 from ghorn/master

enable travis ci
fix for non equidistant control grids
small fix related to NARX models
another old bug fixed related to explicit RK methods and non equidistant control grids
added support for general nonlinear, discrete time models
added support for externally defined nonlinear, discrete time models
Merge branch 'master' of git://github.com/ghorn/acado