Add validation for mappings

First off, this library has been a godsend. Nice work! =D

I was working on a template today, and I added a mapping like this (pardon the grossness...):

```
template.add_mapping("HyperkubeImage", {
"official": {
"v1.1.2": "gcr.io/google_containers/hyperkube:v1.1.2",
"v1.1.4": "gcr.io/google_containers/hyperkube:v1.1.4",
"v1.1.7": "gcr.io/google_containers/hyperkube:v1.1.7",
"v1.1.8": "gcr.io/google_containers/hyperkube:v1.1.8",
"v1.2.0-beta.0": "gcr.io/google_containers/hyperkube:v1.2.0-beta.0"
},
"coreos": {
"v1.1.7-coreos.1": "quay.io/coreos/hyperkube:v1.1.7-coreos.1",
"v1.1.8-coreos.0": "quay.io/coreos/hyperkube:v1.1.8-coreos.0"
}
})
```

The final JSON was generated just fine, but when I tried to pass it to CloudFormation from the AWS CLI, I got the error:

```
A client error (ValidationError) occurred when calling the CreateStack operation:
Template format error: Mappings attribute name 'v1.1.2' must contain only alphanumeric characters.
```

It would be nice if Troposphere could catch these sort of errors prior to rendering the template.