Fix crashes when calculating hash of dir symlinks

`createMapFromPath(dir)` crashes when dir contains (or is) a symlink to a directory. The crash is the result of trying to calculate the hash of a directory.
The reason is: while file walking, directories are filtered but not  symlinks to directories. This pull request add the extra check and associated unit tests. It also filters other non-regular files (FIFO, character devices, etc.).

Reproducing the problem:

    mkdir dir
    ln -s dir symlink

Then

    var versionator = require("versionator");
    versionator,createMapFromPath("dir", function(){}); //No problem
    versionator,createMapFromPath("symlink", function(){}); //Problem
    events.js:72
            throw er; // Unhandled 'error' event
                  ^
    Error: EISDIR, read