Update gitbookInfo.R and checkForGitbook.R

[raulossada]

• On gitbookInfo.R: Using npm list -g instead of gitbook --version since it seems that the later one needs some special permission to execute directly from RStudio in Windows 8.1 x64. Although running cmd and using gitbook --version works fine.
• On checkForGitbook.R: Instead of using npm I propose using npm -v to check if npm is installed, since it returns a status 0 and also its version.
• On initGitbook.R: Code proposed by benmarwick. Sorry I thought that I would be able to submit just the first 2 codes.
• Codes tested on Windows 8.1 x64 - RStudio 0.98.953 - R 3.1.1