Forced floats into doubles

I ran into problems talking to a java rpc server when expecting double's but JS 'number' primitive is serialized as an int.

Scenario 1, correct serialization:
```javascript
require('./lib/serializer').serializeMethodCall('foo', [parseFloat("0.5")]);
```
```xml
<?xml version="1.0"?>
<methodCall>
  <methodName>foo</methodName>
  <params>
    <param><value><double>0.5</double></value></param>
  </params>
</methodCall>
```

Scenario 2, wrong serialization:
```javascript
require('./lib/serializer').serializeMethodCall('foo', [parseFloat("0.5") + parseFloat("0.5")]);
```
```xml
<?xml version="1.0"?>
<methodCall>
  <methodName>foo</methodName>
  <params>
    <param><value><int>1</int></value></param>
  </params>
</methodCall>
```

The neatest thing I could come up with is a Float class that forces number parsing as a double, but it's 5:40am so maybe there's a better solution.

```javascript
require('./lib/serializer').serializeMethodCall('foo', [new xmlrpc.Float(parseFloat("0.5") + parseFloat("0.5"))]);
```
```xml
<?xml version="1.0"?>
<methodCall>
  <methodName>foo</methodName>
  <params>
    <param><value><double>1</double></value></param>
  </params>
</methodCall>
```