change style of main railways (railways=main)


This is the first commit of many for changing the railways. Originally, I was going to use this only for railway=disused, but you (@yohanboniface) stated that you prefer this for the main one instead. 
![hdm-example_rr](https://f.cloud.github.com/assets/955351/1133988/2a36fb60-1be8-11e3-8f49-5eafbe07bf18.png)


Future commits will include: 

- fixing #49 - styling of disused railways. 

- styling railway=other to also used the classic-RR style which is being proposed here (but not as strong. other will have thinner lines this railway

- adding very basic support for bridges. 

I plan on using separate pull requests for those commits. 

Question: what about railway=tram and subway ? I think we should just incorporate that into the 'other' tag.