Added counter and event for exit() syscall

In addition some javadoc comments were added/imporoved, mostly to describe
what the measured statistics actually mean.
Removing processes from model on exit()

When process calls exit() or exit_group() then it will be removed from
ProcessModel. This patch also includes rename of addProcess to
createProcess and similar.

Also, removal from process model means, that it's moved to a different
collection, a collection of terminated processes, which is kept for future
analysis (not yet implemented).
Ability to keep list of opened files per process

Will be used in the future for analysis of what files opened what files.
What needs to be done is to remember if those files were opened for
reading, writing, or both and other interesting flags.