Autoinstall script

## previous attempts
This was mentioned some times already: #414, #144, #59
## info
This PR puts the autoinstall code in a seperated script `autoinstall.vim` which can be sourced directly from Vim. The user can thereby use a simple test to see if Vundle is installed and conditionally source the script.
## ToDo
I am not yet happy with the way I put this in the README if you have good ideas just tell me.
## problem when testing
I noticed that vim is able to source files even with help of the netrw plugin (so the file can be an url). The only problem I have at the moment is to test this script via the github link. If I source my local copy it works and installs vundle. If I try 
```vim
:source https://raw.githubusercontent.com/lucc/vundle/autoinstall/autoinstall.vim
```
my vim uses `elinks -source` to load the script to a temp file and then gives me errors as the tempfile seems to contain html code. Both
```sh
curl https://raw.githubusercontent.com/lucc/vundle/autoinstall/autoinstall.vim
``` 
and 
```sh
elinks -source https://raw.githubusercontent.com/lucc/vundle/autoinstall/autoinstall.vim
```
work fine on the command line. Also my private test script works fine: 
```vim
:source http://luc42.lima-city.de/test
```
(no worries it just containes `echo 'test successful'`)