Update gitbookInfo.R and checkForGitbook.R

* 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`