Remove natural=field rendering

This tag is only used 178 times.

Cross-reference #589
Merge pull request #617 from pnorman/natural_field

Remove natural=field rendering
Only show track and pitch outline at zoom >= 15
Merge pull request #629 from vholten/sportsfields

Only show track and pitch outline at zoom >= 15
Force names of national parks inside polygon

This solves #613.
Switch to a SVG for the parking icon.

Because this is such a simple icon, it's easy to write the SVG for it by hand.

This allows us to only use one symbol file for both parking and private parking, and set the colour in the stylesheet.
Fix parking icon alignment to pixel bounds

The round part of the P was half a pixel off.
Re-factor service widths

* Specify service road widths as variables
* Make service roads on z16 more narrow (solves 3499 on trac)
* Specify service bridge widths in the same way as other service widths
* Remove default value line-cap=butt
* Do not render minor service-tunnels on z14/z15
Add XML prolog to SVG
Flatten out curve of P slightly, for better alignment with pixels.
Drop marker-height, as it duplicates the SVG and can lead to mistakes
Use a more generic name for the transportation blue
Use marker-clip for parking

This avoids duplicate Ps.
Split icon and text colours, since they're different
Use the variable for fuel too
Remove symbol files not actually used in the stylesheet
Remove a couple more files missed
 link "set up your PostGIS database" to docs

it is not so obvious for beginners how to do this
I may confirm that following this instructions resulted in working project on Lubuntu 14.04
Merge pull request #651 from pnorman/symbol_removal

Remove symbol files not actually used in the stylesheet
Merge pull request #655 from mkoniecz/set-up-your-PostGIS-database---documentation

 link "set up your PostGIS database" to docs