make source maps optional

Makes source maps optional:
```
browserify -t coffeeify/no-debug ...
```
Or:
```
var transform =  require('coffeeify');
transform.sourceMap = false;
```
Closes https://github.com/jnordberg/coffeeify/issues/12

One of the tests is failing and it's failing even before these changes. I couldn't understand why it fails. Can you take a look?

```
not ok test/transform.js ................................ 2/3
    Command: "C:\Program Files\nodejs\node.exe transform.js"
    TAP version 13
    not ok 1 adds sourcemap comment including original source
      ---
        file:   stream.js
        line:   79
        column: 10
        stack:
          - getCaller (F:\Tom\Code\GitHub\coffeeify\node_modules\tap\lib\tap-assert.js:418:17)
          - assert (F:\Tom\Code\GitHub\coffeeify\node_modules\tap\lib\tap-assert.js:21:16)
          - Function.equivalent (F:\Tom\Code\GitHub\coffeeify\node_modules\tap\lib\tap-assert.js:182:12)
          - Test._testAssert (F:\Tom\Code\GitHub\coffeeify\node_modules\tap\lib\tap-test.js:87:16)
          - Stream.end (F:\Tom\Code\GitHub\coffeeify\test\transform.js:23:11)
          - _end (F:\Tom\Code\GitHub\coffeeify\node_modules\through\index.js:65:9)
          - Stream.stream.end (F:\Tom\Code\GitHub\coffeeify\node_modules\through\index.js:74:5)
          - Stream.onend (stream.js:79:10)
          - Stream.EventEmitter.emit (events.js:117:20)
          - drain (F:\Tom\Code\GitHub\coffeeify\node_modules\through\index.js:34:23)
          - Stream.stream.queue.stream.push (F:\Tom\Code\GitHub\coffeeify\node_modules\through\index.js:45:5)
          - F:\Tom\Code\GitHub\coffeeify\index.js:88:14
          - compile (F:\Tom\Code\GitHub\coffeeify\index.js:67:5)
          - Stream.end (F:\Tom\Code\GitHub\coffeeify\index.js:85:5)
          - _end (F:\Tom\Code\GitHub\coffeeify\node_modules\through\index.js:65:9)
          - Stream.stream.end (F:\Tom\Code\GitHub\coffeeify\node_modules\through\index.js:74:5)
          - ReadStream.onend (_stream_readable.js:483:10)
          - ReadStream.g (events.js:180:16)
          - ReadStream.EventEmitter.emit (events.js:117:20)
          - _stream_readable.js:920:16
          - process._tickCallback (node.js:415:13)
          -
        found:
          version:        3
          file:           F:\Tom\Code\GitHub\coffeeify\example\foo.coffee
          sourceRoot:
          sources:
            - F:\Tom\Code\GitHub\coffeeify\example\foo.coffee
          names:
            -
          mappings:       AAAA,OAAO,CAAC,GAAR,CAAY,OAAA,CAAQ,UAAR,CAAZ,CAAA,CAAA
          sourcesContent:
            - console.log(require './bar.js')
        wanted:
          version:        3
          file:           F:\Tom\Code\GitHub\coffeeify\example\foo.coffee
          sourceRoot:
          sources:
            - F:\Tom\Code\GitHub\coffeeify\example\foo.coffee
          names:
            -
          mappings:       AAAA,OAAO,CAAC,GAAR,CAAY,OAAA,CAAQ,UAAR,CAAZ,CAAA,CAAA
          sourcesContent:
            - console.log(require './bar.js')
        diff:   |
          {
            "version" : 3,
            "file" : "F:\Tom\Code\GitHub\coffeeify\example\foo.coffee",
            "sourceRoot" : "",
            "sources" : [
              "F:\Tom\Code\GitHub\coffeeify\example\foo.coffee"
            ],
            "names" : [

            ],
            "mappings" : "AAAA,OAAO,CAAC,GAAR,CAAY,OAAA,CAAQ,UAAR,CAAZ,CAAA,CAAA",
            "sourcesContent" : [
              "console.log(require './bar.js')
          " // != "console.log(require './bar.js')
          "
            ]
          }
```