Added Than's blog post url about compiling TrenchBroom on Linux to the Linux build instructions.
Added link to website and downloads to the Readme.
Updated contributors in about box.

Signed-off-by: Kristian Duske <kristian.duske@gmail.com>
Improved performance of loading models and textures. Added documentation about "Select by Line Numbers" command.
Draw thinner grid lines for small grid sizes.

Signed-off-by: Kristian Duske <kristian.duske@gmail.com>