branches/tags/commit hashes support

I have added support for git tree-ish. Branch, tag or commit can be specified now. 
```vim
Bundle 'vendor/bundlename', 'feature/branch-name'
Bundle 'vendor/bundlename', 'a83commit75hash2d5hd1bsdhd'
Bundle 'vendor/bundlename', '0.9.1'
```

Also i've added alias support in case there are compatible bundles with same name by different vendors
```vim
Bundle 'vendor/bundlename'
Bundle 'othervendor/bundlename as altname'
```

and local flag to prevent vundle from managing bundle installs and updates, but still registering it's resources. Useful for developers.
```vim
Bundle 'bundlename', {'local': 1}
```
new Bundle format is backwards compatible and can be described by
``` 
Bundle '(git url)|([vendor/]name)[ as alternate_install_name]'[, 'tree-ish'][, {json encoded options}]
```
Related issue: #35