fix iterating through paths that have spaces in them

Fixes #344, #196
add tests for exec
fix exec fails for invalid version
hack around the hack around bash `echo`
test completions for exec
add tests for completions
improve detection of completion support for commands

Enable JavasCript, Lua and Erlang scripts to provide completions
saner assertions for multiline output
simplify iterating through arglist
un-indent shebang for test executables
better emulate `ruby -S` behavior in testing

Per https://github.com/ruby/ruby/blob/7d3db3c/ruby.c#L1383-1391
Merge branch 'master' of github.com:sstephenson/rbenv into develop

* 'master' of github.com:sstephenson/rbenv:
  better emulate `ruby -S` behavior in testing
  un-indent shebang for test executables
  simplify iterating through arglist
  saner assertions for multiline output
  improve detection of completion support for commands
  add tests for completions
  test completions for exec
  hack around the hack around bash `echo`
  fix exec fails for invalid version
  add tests for exec
  fix iterating through paths that have spaces in them