These changes add in ulimit for file descriptors.

For #56
Making default for ulimit_file nil.
It'll only apply if you set it.
For #56.
Remove `non-nil` checks for `ulimit_file` option

Not really necessary. If one set’s it to `false`, it’s their problem.
Add int. test(s) for `ulimit_file` option

Definitely not the prettiest tests I’ve written, but they do the job.
Merge branch 'solarce-ulimit-support'