Model generator fails for camel case standard types

Generating a model from a camel case standard type generates incorrect filenames and class names.

i.e
SiteHistory creates the following

``` Ruby
sitehistory.rb

class Sitehistory < ActiveForce::SObject
end
```

Should instead be the following

``` Ruby
site_history.rb

class SiteHistory < ActiveForce::SObject
end
```
