Tests for handling of special chars in filenames

This exposed bad path escaping in combine_options and format.
Fix missing path shell escaping

This fixes issue #45.
Refactor shellescaping to happen in a single place

Adjusted one test because + char should be escaped.
Also removed dependency on OS detection, because Ruby should properly
escape for the OS it's running on. Replaced manual path joining with
File.join for the same reason.
Merge pull request #136 from felixbuenemann/fix-shell-escaping

Fix shell escaping