Add unit tests for CAWS and SPAS15 ammo
Disable Shift as replacement for Alt (Option) on Linux and Mac OS X

Shift was used as replacement for Alt to allow moving backward (issue #40), but
it turned out not to be a good solution.

Now to move backward use:
- Option + Left button mouse (LBM) on Mac OS X
- Alt + LBM or Alt + Meta (WinKey) + LBM on Linux, depending on your desktop environment

This fixes issue #123 and reverts changes made for issue #40.

If there are still users of one-button Mac mouse, you are out of luck, sorry.
You should start the game with flag -no3btnmouse, that will allow you to use
Command + mouse click as replacement for right mouse button, but you won't be able
to move backward.
add basic unit test for weapons
Merge branch 'bug120' into weapon_unit_testing
fix unit tests, which doesn't compile anymore

4 unit tests doesn't pass because of bug #120
[  FAILED  ] 4 tests, listed below:
[  FAILED  ] Items.bug120_cawsAmmo
[  FAILED  ] Items.bug120_12gAmmo
[  FAILED  ] Items.bug120_cawsDefaultMag
[  FAILED  ] Items.bug120_spas15DefaultMag
-- BUCKSHOT and REGULAR were exchanged
fix ammo types, issue #120

Now there are only two tests which don't pass:
[  FAILED  ] 2 tests, listed below:
[  FAILED  ] Items.bug120_cawsDefaultMag
[  FAILED  ] Items.bug120_spas15DefaultMag
Fixing issues #120, #124 (incorrect ammo type for CAWS and SPAS-15)
Merge branch 'bug120'