Add support for Cygwin-style absolute paths on Windows Systems

[semperos]

This pull request addresses Issue #37

Obviously it is customary to run CoffeeScript under Node.js. For Windows users, it is common to compile Node.js with Cygwin in order to use CoffeeScript. However, Cygwin expects it's own style of absolute paths (prefixed with /cygdrive/C), which of course expand-file-name does not expand to. Given these restrictions, at the present time coffee-mode's file compilation functionality does not work on Windows.

I've implemented three things:

• A coffee-cygwin-mode customizable var, for enabling/disabling support for Cygwin-style paths on Windows for CoffeeScript commands
• A coffee-cygwin-prefix customizable var, to allow people to set /cygdrive/C to whatever their Cygwin expects to see (dependent on installation)
• A coffee-cygwin-path function, which given an expanded file name will return a Cygwin-style absolute path + file name

Finally, I tweaked coffee-command-compile to leverage these settings when the system-type is windows-nt.

Without these changes, this mode's file compilation functionality does not work on Windows for the "canonical" setup of CoffeeScript on Node.js.

[semperos]

Second commit encapsulates the system check and Cygwin path support in a single function, coffee-universal-path. Returns the regular expanded file name for all other systems, like this mode already does.