Rename simple_auth -> rauth

Task 13352 - rename simple auth -> rauth

## Changes
* The main job is done with sed: `find . -type f | egrep -v 'git|metric_fu' | xargs sed -i 's/SimpleAuth/Rauth/g; s/simple_auth/rauth/g; s/SIMPLE_AUTH/RAUTH/g;'` verified with `git diff`.
* Remove `ColorizedText` module (we use it only in `.simplecov`), use `colorize` gem instead
* Update README

## RSpec
```
Finished in 3.23 seconds
43 examples, 0 failures

Randomized with seed 47225

Coverage report generated for RSpec to /home/blake/dev/work/tmx/rauth/coverage. 121 / 121 LOC (100.0%) covered.

Coverage: 100.0% is over the threshold: 100.0%.
```

## Doc
```
yard --list-undoc
[warn]: Unrecognized/invalid option: --list-undoc
Files:          10
Modules:         9 (    0 undocumented)
Classes:         5 (    0 undocumented)
Constants:       1 (    0 undocumented)
Methods:        26 (    0 undocumented)
 100.00% documented
```