Improve handling of multiline "var" declarations

I'm looking into ways to improve the handling of `var ()` declarations like these ones:

There are a number of issues here:

![screen shot 2014-08-22 at 11 33 15 am](https://cloud.githubusercontent.com/assets/17565/4013584/d4a8cfa4-2a17-11e4-9f92-a3b5664eb256.png)

![screen shot 2014-08-22 at 12 00 26 pm](https://cloud.githubusercontent.com/assets/17565/4013588/e68b116e-2a17-11e4-855e-b35f52f6d286.png)

 * The closing `)` isn't highlighted (not as obvious with this syntax theme).
 * Initialization expressions on the right-hand sides of assignments are not highlighted correctly.
 * If you add a `var (` expression in an existing file, the rest of the file will be highlighted incorrectly until you put the closing `)` on its own line and re-trigger highlighting.
 * Types are lexed as variable names, even in the simpler `var x int`-style declarations.

The relevant part of the Go spec is [here](https://golang.org/ref/spec#Variable_declarations).

Interestingly, `const` and `import` don't seem to have the same issues.

re #27