Refactor ReadSnippetFiles() and s:Glob()
Fix snippet name collision bug

This was introduced by d42783a. Fixes #122.

Also that last commit message should have said GetSnippetFiles, not
ReadSnippetFiles.
Restore performance benefits of old s:Glob
Fix SnipMateOpenSnippetFiles

In particular the command didn't work when a path in snippet_dirs used
tilde expansion.
Fix glob() for use with older Vim versions

The optional third argument was added in Vim 7.3.465.
Release version 0.86