Fix npm issue #3380.

As discussed I created a fix for the above mentioned issue. I would very much appreciate any feedback, on issues of style and/or naming conventions since this is my first delve into node. I hope you see some value in these changes and accept this request.

Fix npm issue #3380 by changing the following:
- Adapt the regular expression used to match the first
line.
- Create a simple module that can convert shell style variable
declarations: `NODE_PATH=./lib:$NODE_PATH` to the equivalent batch syntax:
`@SET=NODE_PATH=./lib:%NODE_PATH%`. Furthermore the structure of the generated
shim now looks like this:
````
    @SETLOCAL
    <variable declarations>
    <original content>
    @ENDLOCAL
````
  - Note that the new segments are only added to the file if there were any
variable declarations.

- The generated shell script carries over the captured variable declaration as
is.
- Add some extra tests to validate the behavior.
- Remove some unnecessary white-space in the generated shim.