Add 'File Diff with Previous' feature and bug corrections

Hi,

I corrected two bugs and add a new feature:

* Add unicode to menu strings (Correction of Issue #48)

* Add window support for running external tool (Correction of Issue #47)

* Add `File Diff with Previous` feature. I was inspired by [WinMerge plugin](https://github.com/SublimeText/WinMerge). Select the first file and then the second file to compare.

I used the `master` branch (not the `st2` one) and all the feature are working fine. I am using ST 2.0.2.2221 on WinXp. I do not know what the difference are between the two branches. I chose the one with the latest commit.