Merging feed and rss crates

[chrisppy]

This issue is to merge the functionality of feed into rss.

Port the following pieces of feed:

• [ ] Builder with Validate methods
• [ ] Getter methods
• [ ] FromUrl trait
• [ ] Validate Trait
• [ ] rustfmt check in travis

Change to the following:

• [ ] Structs have private fields - reason noted here

• [ ] Update documentation

If I am forgetting anything, I will update this as I am working on the above.

[chrisppy]

resolved by #35