Add tmpdir method

This lets us choose the best tmpdir information depending on the Ruby
and the environment.

For #2022
Use LogStash::Environment.tmpdir for better MRI support

For #2022
Fix specs under MRI 2.1.2

For #2022
Improve MRI support. Also make 'install' accept multiple plugins to install.

For #2022 and #2023
Ensure all ruby files have an 'encoding: utf-8' header.

This was done with this oneliner:

    grep -L 'encoding:' {spec,lib}/**/*.rb | xargs sed -r -i -e '1i # encoding: utf-8'

For #2022