Fixing -c config option.

This fixes an issue with the -c (--config) option where the specified config file's path would be resolved from the cli.js directory path instead of process execution directory path, causing an error.

For example, if the user is in `~/project-dir/` and runs `eslint -c special-config.json lint-me.js`, the CLI would try to require `special-config.json` from `~/project-dir/node_modules/eslint/lib/special-config.json` instead of the intended `~/project-dir/special-config.json`.

Changing `__dirname` to `process.pwd()` resolves the issue.
Fixes issue with fs.existsSync on NodeJS 0.6

Fixes an issue where`lib/cli.js` was using `fs.existsSync`, which prior to NodeJS 0.8 was `path.existsSync`. So, on NodeJS 0.6, the CLI would throw an error.
Merge pull request #89 from iancmyers/fix-config-option

Fixing -c config option in CLI