[Proj] Support for Modified Krovak projection

[Michal Seidl]

Hello,
I have implemented in my local fork of proj source code Modified Krovak 
projection. I have given krovak-m proj.4 code for this projection.

It can be used to fix missing EPSG codes 5221,5224,5225... in epsg 
transformation file.

If the proj community is interested or someone else is already working 
on it please let me know how can I correctly contribute to project.

Michal S.