Merge defaults, request config, and instance config in request method and add support for defaults.baseURL

[nickuraltsev]

Fixes #175

[mzabriskie]

As I was looking over your PR I realized that there is a fundamental problem with the way that defaults/config has historically been implemented. I started trying to describe the problems as I saw them, but ultimately decided that I could show what I meant more easily with a diff. Take a look at my commit and let me know what you think. 82847f7

In short:

• I removed any reference to defaults outside of lib/axios
• Moved transformData and merging headers out of adapters into Axios.prototype.request
• Establish an order of precedence to be defaults, this.defaults, config

I was able to catch some existing buggy behavior while I was writing tests for this refactor and I think this will be more stable all around.

If you think that this approach makes sense I will merge my branch.

[nickuraltsev]

Your commit looks great. It's definitely a better way to solve the issue with defaults than #179.

[mzabriskie]

Thanks for the feedback. Also I appreciate your PR.