Allow gases to travel through fences, wire gates, bars [WIP]

As per the title says. I did not test this... but what can go wrong??? I changed so little! :D 

I've added a PERMEABLE tag to terrain that allows gases to flow through. Danzer suggests allowing blobs to travel through these; maybe this will be a future use for the permeable tag.

This was brought up my IRC members MShock777 and Atzerkins!

To do : 
- Properly add the flags, so this actually compiles
- Document the flags properly
- Disallow fields spreading to other fields if move_cost is not greater than 0
- Consider allowing gases to leak under doors
You're right that it's missing some parentheses and checks, but I'm not sure why you're checking for tmpfld and density so many times.
Candidates are, "existing weaker fields", "no field plus navigable" and "no field plus permeable"
This can be refactored to:
```
if( ( tmpfld && tmpfld->getFieldDensity() < cur->getFieldDensity() ) || 
    ( !tmpfld && ( move_cost( x + a, y + b ) > 0 ||
                   has_flag( "PERMEABLE", x + a, y + b ) ) ) ) {
```
Which as you may note is just a matter of taking the existing "move_cost( x + a, y + b ) > 0" clause and replacing it with "(move_cost( x + a, y + b ) > 0 || has_flag("PERMEABLE", x + a, y + b) )".