Add explicit_nil_handling option to codebase

Fixes issue 33 which requests that there be an option to specify how Builder treats nil values from initialization. Without specifying the explicit_nil_handling option as true, the codebase functions the same way. 

```ruby
    b = Builder::XmlMarkup.new 
    b.tag! "foo", "bar"
```
 yields an output of 
```xml
<foo>bar</foo>
```
as does
```ruby
    b = Builder::XmlMarkup.new explicit_nil_handling: false
    b.tag! "foo", "bar"
```
However, when a tag has a nil value, and the option is turned on, it adds a nil="true" attribute to the tag, in order to match the way Rails explicitly denotes nil values when rendering ActiveRecord objects as xml.
```ruby
    b = Builder::XmlMarkup.new explicit_nil_handling: true
    b.tag! "foo", nil
```
yields
```xml
<foo nil="true"/>
```

commit 430b9762c436f180db8686407b5002cb4f71e8b9
Author: Dermot Haughey <hderms@gmail.com>
Date:   Tue Jan 15 19:36:00 2013 -0600

    Add test

commit b9adb4d916ae1cf897ea2c1f39f5ad0c0def4460
Author: Dermot Haughey <hderms@gmail.com>
Date:   Tue Jan 15 19:35:50 2013 -0600

    Cleanup of code

commit 8e8a7873b48e956e4642d31265dc767b5fafb096
Author: Dermot Haughey <hderms@gmail.com>
Date:   Tue Jan 15 19:25:42 2013 -0600

    Make changes to add option