Used static_assert for static assertion

Yay for C++11! static_assert is available since GCC 4.3.

Fixed warning when compiling with GCC 4.8:

src/utils/mathutils.cpp:53:18: warning: typedef ‘float_must_be_32_bits’
locally defined but not used [-Wunused-local-typedefs]
Fixed error message

Thanks to Ablu for noticing.
Fixed small script documentation error
Forgot to rename npc_message in some docs
Removed old monster attributes
Simplified BeingComponent::updateDirection

Same as in the client.
Fixed crash when checking whether a character is already online

Not every connected game client has a character in the world. The code
was checking this, but since commit aa04597c5f8bb806996 the pointer was
being used before being checked.
Fixed typo in script docs
Added a few file to .gitignore
Use nullptr instead of NULL everywhere
Merge branch 'master' into lpc2012