Renames SAI to DAI and SKR to PETH.

[MicahZoltu]

This PR was generated via the following operations:

1. Case sensitive find and replace in all files: sai -> dai
2. Case sensitive find and replace in all files: SAI -> DAI
3. Rename files/folders with sai in the name to have dai in the name instead.
4. Case sensitive find and replace in all files: skr -> peth.
5. Case sensitive find and replace in all files: SKR -> PETH.
6. Rename files with skr in the name to have peth in the name instead.
7. Delete the line at the top of the readme indicating that SAI is really DAI and SKR is really PETH.

Unfortunately, for some of the smaller files, git sees enough of the file changed to not notice that it was a rename/move and thinks it was a deletion/addition, so they don't line up when reviewing. Locally I believe you can lower git's threshold and see if it can display diffs instead of additions/removals.

[MicahZoltu]

I did try to review the find and replace changes to make sure they didn't pick up some word with sai in the middle. I'm reasonably confident there were no such cases, but someone more familiar with the codebase should scan through the changes and verify.

[nmushegian]

I think we want to keep "sai" at the dev level, but SKR->PETH in the documentation might be a good change

[nmushegian]

I guess the README should reflect http://developer.makerdao.com/dai/1/api/

Can we first process a PR that only changes documentation? This means some places you'll need e.g. "to get PETH balance you call tub.skr()

[MicahZoltu]

Personally, I think it would be even more confusing if the Readme mentions DAI/PETH but the source code all mentions SAI/SKR. My recommendation is to pick a naming convention and stick with it and be thorough in your transitions (e.g., rename it everywhere if you are going to rename it).

What is the argument for leaving the code with SAI/SKR while all other documentation and user-facing material is DAI/PETH?

[MicahZoltu]

Maybe the right answer is to just publicly rename DAI to SAI and PETH to SKR?