Semicolons added to transformed statements

When source contains no semicolons, just newlines. Transforming one statement into another inserts semicolons that weren't previously there:

```js
var prog = recast.parse("var m = require('module')\n var x");

prog.program.body[0].type = "ImportDeclaration";
prog.program.body[0].specifiers = [{type: "ImportDefaultSpecifier", local: {type: "Identifier", name: "m"}}];
prog.program.body[0].source = {type:"Literal", value: "module", raw: '"module"'};

console.log(recast.print(prog).code);
```

Expected output:

```
import m from "module"
var x
```

Actual output:

```
import m from "module";
var x
```

I think this will hinder the use of recast by people who have their source code formatted without semicolons. I think this also ties into issue #13 - there currently is no option to suppress semicolons.

Start by running the JavaScript reproduction in the issue and compare the printed output with the expected semicolon-free form. Trace the transformed statement through recast's printing or pretty-printing path; done means transformed statements preserve the source's lack of semicolons without adding one before the following declaration.

Written by the indexing model from the issue text.