Fix keg method visibility

Only link_dir needs to be protected
Give Keg#link_dir parameter a more descriptive name
This was troubling me for some time now...macvim insist in not linnking with the python inside the path variable. This is a fix for this and a remove from old code that didnt work. Try yourself. Works for homewbrewed python and other pythons (such as anaconda). A must if you want vim-python+macvim+anaconda.
macvim: add 7.4-73 bottle.