Fixed pushing repo to GitHub for *nix boxes

The necessary Git commands to init a git repo in the new_report directory are now fixed (for os x / linux / unix) but I need to invoke shQuote to ensure quoting of commands is correct for windows. All git commands must be issued inside the directory of the local repo. By default the R `system()` function issues commands in _/bin/sh/_ on *nix systems. So _cd_ to the repo directory and chain git command using *&&* operator on the command line (which works on windows and *nix). 

I also added a new option entitled "last.report" at the end of `new_report()` which sets the option to the name of the folder used in new report. `repo2github()` then uses this option as the default repo name, which I feel is safer than using the current working directory. It also reduces the number of items a user has to specify before they can push their repo to the clouds. In fact if github.user is set beforehand the user will only need to specify their github password to push the report directory to GitHub.

I also changed the folder creation from your custom `folder()` function to `dir.create()` to allow a user to specify a folder path that doesn't yet exist using the recursive option. Perhaps recursive can be integrated into folder though?