title: error in saveGIF command in animation package
body: <p>I'm trying to use for the first time <code>animation</code> package in R.</p>
  
  <p>I'm using this simple code:</p>
  
  <pre><code>saveGIF({
      for (i in 1:10) plot(runif(10), ylim = 0:1)
  })
  </code></pre>
  
  <p>But R shows this error (that i've translated in english):</p>
  
  <pre><code>Executing: 
  "convert" -loop 0 -delay 100 Rplot1.png Rplot2.png Rplot3.png Rplot4.png
      Rplot5.png Rplot6.png Rplot7.png Rplot8.png Rplot9.png Rplot10.png
      "animation.gif"
  "convert" -loop 0  -delay 100 Rplot1.png Rplot2.png Rplot3.png Rplot4.png Rplot5.png Rplot6.png Rplot7.png Rplot8.png Rplot9.png Rplot10.png "animation.gif" is not recognized like an internal or external command,
   an executable program or batch file.
  Parameter not valid - 0
  an error occurred in the conversion... see Notes in ?im.convert
  [1] FALSE
  Warning messages:
  1: running command 'C:\Windows\system32\cmd.exe /c "convert" -loop 0  -delay 100 Rplot1.png Rplot2.png Rplot3.png Rplot4.png Rplot5.png Rplot6.png Rplot7.png Rplot8.png Rplot9.png Rplot10.png "animation.gif"' had status 1 
  2: In cmd.fun(convert) :
    '"convert" -loop 0  -delay 100 Rplot1.png Rplot2.png Rplot3.png Rplot4.png Rplot5.png Rplot6.png Rplot7.png Rplot8.png Rplot9.png Rplot10.png "animation.gif"' execution failed with error code 1
  3: running command '"convert" -loop 0  -delay 100 Rplot1.png Rplot2.png Rplot3.png Rplot4.png Rplot5.png Rplot6.png Rplot7.png Rplot8.png Rplot9.png Rplot10.png "animation.gif"' had status 4 
  4: In normalizePath(path.expand(path), winslash, mustWork) :
    path[1]="animation.gif": Impossible to find the file specified
  </code></pre>
  
  <p>Where is the problem?</p>