Add unit tests

Closes #42
There's a lot of complexity in this function. Took a shot at collapsing some of the conditions, but likely missed some. Main goal was to name each of the conditions, but I couldn't determine one of the branches. See below:

``` vim
function! SetupCommand()
  if s:isMacGUI()
    let s:rspec_command = "silent !" . s:plugin_path . "/bin/" . g:rspec_runner . " '" . s:rspec_command() . "'"
  elseif s:isWindows()
    let s:rspec_command = "!cls && echo " . s:rspec_command() . " && " . s:rspec_command()
  elseif " what conditions should trigger this branch?
    let s:rspec_command = "!clear && echo " . s:rspec_command() . " && " . s:rspec_command()
  else
    let s:rspec_command = g:rspec_command
  endif
endfunction

function! s:isWindows()
   return has("win32") && fnamemodify(&shell, ':t') ==? "cmd.exe"
endfunction

function! s:rspec_command()
  if exists("g:rspec_command")
    return g:rspec_command
  else
    return "rspec {spec}"
  endif
endfunction
```