Fix variable typo in source code
Merge pull request #133 from bsutic/fix-typo

Fix variable typo in snippet ftplugin
Fix typos in mapping documentation
Use 'sw' if 'sts' is non-positive

At some point, Vim started allowing negative values for 'softtabstop', resulting in hard tabs being removed entirely from expanded snippets. Instead, correctly use 'shiftwidth'.
Add g:snipMate_no_default_aliases option

Fixes #137
Include instructions for installing vim-snippets