Combine enemy and enemygroup layers

Closes #1089

Changes:
- the old 'enemy' map layer has been removed; the 'enemygroup' layer was
renamed to 'enemy' to replace it
- filenames are no longer used for placing enemies. Instead, use enemy
categories and the level property of groups to place specific enemies
- group level no longer requires a maximum value. The minimum is used
when it's missing
- the group area defaults to 1x1 if not defined
- the number of enemies in a group is 1 by default
- waypoints and wander_area only work for single enemies
- direction specifies the starting direction of the entire group
- requires_(not_)status now applies to enemy groups
Don't allow out-of-bounds values for variables in enemy groups
Add SDL2 to the INSTALL file

Tested on debian and fedora systems, the instructions for arch and windows
are untouched.
adapt .travis.yml continuous build integration for sdl2
Revert "adapt .travis.yml continuous build integration for sdl2"

travis doesn't have sdl2 yet apparently.

This reverts commit 99544132666572e90aa4b4e09e3ad7becf685d1f.
Run travis with sdl 1.2
Merge pull request #1094 from stefanbeller/readme

Add SDL2 to the INSTALL file
Replace wander_area with wander_radius

Changes:
- All enemies wander by default with a radius of 4 tiles.
- Since an enemy can either have set waypoints or wander randomly (but
never both), wander-specific code has been removed. Instead, wander
points are added to the waypoint queue. The pause between wandering is
the same as waypoint_pause.
Fix bug where wandering enemies would base pathfinding off player pos
Bypass enemy group level if min & max levels are zero
Fix wandering enemies that can see the player through walls
Merge pull request #1092 from dorkster/map_enemy

Combine enemy and enemygroup layers