title: Error when saving xlsx workbook with package openxlsx
body: <p>I'm going through the vignette example in the openxlsx package in R Statistics, but I can't save the workbook and I can't figure out why. The code below is a modified example from the vignette, just to keep it simple.</p>
  
  <pre><code>setwd("c:/users/kenneth/documents/r/2014-04-29_openxlsx")
  require(openxlsx)
  require(ggplot2)
  wb &lt;- createWorkbook()
  addWorksheet(wb, sheetName = "Motor Trend Car Road Tests", gridLines = FALSE)
  addWorksheet(wb, sheetName = "Iris")
  addWorksheet(wb, sheetName = "Conditional Formatting")
  saveWorkbook(wb=wb, file="basics.xlsx", overwrite=TRUE) ## save to working directory
  </code></pre>
  
  <p>The error message I get is:</p>
  
  <pre><code>Warning message:
  running command '"zip" -r1 "c:/users/kenneth/documents/r/2014-04-29_openxlsx/basics.xlsx" "[Content_Types].xml" "_rels" "docProps" "docProps/app.xml" "docProps/core.xml" "xl" "xl/_rels" "xl/_rels/workbook.xml.rels" "xl/charts" "xl/drawings" "xl/drawings/_rels" "xl/printerSettings" "xl/printerSettings/printerSettings1.bin" "xl/printerSettings/printerSettings2.bin" "xl/printerSettings/printerSettings3.bin" "xl/styles.xml" "xl/tables" "xl/tables/_rels" "xl/theme" "xl/theme/theme1.xml" "xl/workbook.xml" "xl/worksheets" "xl/worksheets/_rels" "xl/worksheets/_rels/sheet1.xml.rels" "xl/worksheets/_rels/sheet2.xml.rels" "xl/worksheets/_rels/sheet3.xml.rels" "xl/worksheets/sheet1.xml" "xl/worksheets/sheet2.xml" "xl/worksheets/sheet3.xml" ' had status 127 
  </code></pre>
  
  <p>This is my sessioninfo:</p>
  
  <pre><code>R version 3.1.0 (2014-04-10)
  Platform: x86_64-w64-mingw32/x64 (64-bit)
  
  locale:
  [1] LC_COLLATE=Danish_Denmark.1252  LC_CTYPE=Danish_Denmark.1252   
  [3] LC_MONETARY=Danish_Denmark.1252 LC_NUMERIC=C                   
  [5] LC_TIME=Danish_Denmark.1252    
  
  attached base packages:
  [1] stats     graphics  grDevices utils     datasets  methods   base     
  
  other attached packages:
  [1] ggplot2_0.9.3.1 openxlsx_1.0.3 
  
  loaded via a namespace (and not attached):
   [1] colorspace_1.2-4 digest_0.6.4     grid_3.1.0       gtable_0.1.2    
   [5] MASS_7.3-31      munsell_0.4.2    plyr_1.8.1       proto_0.3-10    
   [9] Rcpp_0.11.1      reshape2_1.4     scales_0.2.4     stringr_0.6.2   
  [13] tools_3.1.0   
  </code></pre>
  
  <p>I have updated to the most recent version of R, devtools and Rtools.</p>