doc: mention the potential need to install ruby-dev/ruby-devel etc

I'm not going into specifics here because there are too many platform
variations possible, but I got an email from a user stating that he
needed to do:

  sudo apt-get install ruby-dev

And that the error message he got didn't make this obvious:

  'require': no such file to load — mkmf (LoadError)

(I might also consider catching that `LoadError` and trying to print
something more helpful.)
Print helpful message if "mkmf" require fails

Or at least hopefully a bit helpful...
Avoid hang on Windows with g:CommandTTraverseSCM != "pwd"

The `while` loop in `PathUtilties#nearest_ancestor` was never
terminating on Windows because the root of the filesystem is not "/" but
rather "C:/" (or some similar drive letter).

The original fix implemented here was to replace:

  return nil if path == '/'

with:

  return nil if path == '/' || path =~ %r{\A[A-Z]:[/\\]\z}

This looks for any letter, followed by a colon, followed by "/" or "\".
In my testing on a single Windows VM (running XP and Ruby 1.9) I found
that "/" is used, but I wanted to be a bit more liberal in what I accept
here in order to minimize the risk of running into an infinite loop on
other versions of Windows, or other versions of Ruby.

In the end, however, I went with the more future-proof fix of checking
for whether the loop has advanced.

Fixes:

https://github.com/wincent/command-t/issues/110
Handle 'hidden' unwanted split edge case

If you had `'hidden'` off and a modified buffer in two windows, the old
logic here would run `:sp` instead of the `:e`, causing an unwanted
split.

We used `:sp` in order to avoid seeing an error message from Vim, but it
turns out that in this case Vim won't actually show an error because it
considers the extra window containing the modified buffer to mean that
you're not actually abandoning the buffer with unsaved changes.

The fix here detects that scenario by counting windows with the current
buffer in them. Note that this won't consider windows in other tabs,
because Vim doesn't expose those at all through `VIM::Window`.

Fixes:

https://github.com/wincent/command-t/issues/124