Feature native deps

This fixes #85.

Currently we don’t support `nativeDependencies` with platform dependent versions.

I’m thinking about the following forms.

**normal, platform independent:**
```json
{ "nativeDependencies": { "net.iamyellow.tiws": "*" } }
```

**platform specific, one version:**
```json
{ "nativeDependencies": { "net.iamyellow.tiws": { "ios": "*" } } }
```

**platform specific, multiple versions:**
```json
{ "nativeDependencies": { "net.iamyellow.tiws": {
  "ios": "*",
  "android": "*"
} } }
```

If it’s ok for you I’ll implement it this way for widgets too.