Automatic chdir

As promised I fixed the feature to actually work fine, fixed the repo, and also completed the documentation.

This commit allows to change the working directory of vim according to the directory of the build file. This way you do not have to use the MakeshiftBuild command and this plugin works nice together with other  plugins such as vim-dispatch.

I thought the makeshift_root does not have to be a global variable, however I didn't touch the documentation on that matter -- if you decide not to keep this change, it does not brake the other functionality.