remove genid() from util

See #13.
move gl_strerror() to draw

Close #13
move gfx functions to draw

See #12.
move gfx tests to draw suite

See #12.
(re)write some more tests while we are at it

See #12.
move physics functions to entity

See #12
move physics tests to entity suite

See #12.
fix headers

See #12.
rename 'sys' module with event related functions to 'event'

See #12.
fix includes after renaming, get rid of 'running' variable

See #12.
Merge branch 'restructurize' into develop

Close #12.

* restructurize:
  fix includes after renaming, get rid of 'running' variable
  rename 'sys' module with event related functions to 'event'
  fix headers
  move physics tests to entity suite
  move physics functions to entity
  (re)write some more tests while we are at it
  move gfx tests to draw suite
  move gfx functions to draw
  move gl_strerror() to draw
  remove genid() from util