Support mingw-compiler on Windows

Since Matlab2014a, the mexopts.bat cannot be used anymore to set the mex-compiler to cygwin, rendering it pretty much useless for most recent versions. However, Mathworks now has a MinGW-addon (http://de.mathworks.com/help/matlab/matlab_external/install-mingw-support-package.html) that we should support. For this, we need to

1. Allow for a mingw command line flag
2. Adjust dyn_mex with the respective case distinction and necessary flags
3. Update http://www.dynare.org/DynareWiki/ConfigureMatlabWindowsForMexCompilation