Convert package dash

Addresses #8 by converting invalid characters in package names to underscores.  You may prefer a different approach, let me know if you want any changes made.

```
$ cat somethrift-v1.thrift
struct Result {
    1: optional binary value
}

$ cat somethrift.go
// This file is automatically generated. Do not modify.

package somethrift_v1

import (
	"fmt"
)

type Result struct {
	Value []byte `thrift:"1" json:"value"`
}
```