Fixes #54, thanks @zmoazeni!

Always return an integer when asking for stack sizes because "when we
convert the nil to an int in C bad things happen"...
Remove `init` option to Byebug.start

This option was used to enable/disable saving information about program
path and arguments to make restarts possible. This feature has been
removed and this information is always saved now. This also fixes some
order dependent failures in the `restart` command tests.