Cleanup package synopsis.

Here I've modified `BlankLineStrippingWriter` to take in a start offset, a literal number of lines to write without stripping blank lines.

Then we hard-code the offsets found in the template files to the Go package statement (17/3/3).

This allows us to have cleaner package synopsis:

* Before: http://i.imgur.com/80f2lVC.png
* After: http://i.imgur.com/tZ1zfhI.png

This fixes #39.
I agree with @capnm that, while this method produces awesome package descriptions, it is particularly fragile. Maybe we can implement something like a "start stripping" marker that allows us to avoid hard-coding line counts? Or maybe we can implement a marker that says "keep this blank line but remove the marker" (e.g., `///` is kept and transformed to `//` while plain `//` or `\n` is not).