Add historical changelog and mechanics to automatically generate it via 'rake release'

See this for the skeleton format I went with: https://github.com/jrafanie/linux_admin/blob/add_changelog/CHANGELOG.md


From the second commit:
```
The normal workflow should be:
1) Make your linux_admin code changes.
2) Test and commit these changes.
3) When ready for a new tag...
4) Run rake release
5) This will invoke the update_changelog rake task, it will ask you to:
6) Update LinuxAdmin::VERSION.
7) Verify that the script correctly added the commits since the last tag to the beginning of the CHANGELOG.md.
8) Add the correct version information before these commits.
9) Commit these changes
10) Hit enter and rake release will be run to create the tag, and push the version to rubygems.org.
```