Make spec_helper manage tmpdir.

This adds management of the ENV[TMPDIR] when running rspec tests using
spec_helper. Before tests are being run, a temporary directory is
created under the current tmpdir. This temporary directory is then set
as the ENV[TMPDIR].

After all tests have been run, this temporary directory is removed.
The directory is created with the prefix "rspecrun" to easily identify
it in case the spec testing crashes before having cleaned up.

Conflicts:

spec/spec_helper.rb