fix #177, add a lock file for n

before the version installed, touch a file named `n.lock` in version directory, and util the version has been installed/downloaded locally, remove the file.

Then for that checking function, just check the n.lock of the corresponding version directory is existed.
/cc @tjwebb
on `-a`: http://www.tldp.org/LDP/abs/html/fto.html

> This is identical in effect to -e. It has been "deprecated," [1] and its use is discouraged.