Bump version to 0.0.3
Add installation instruction using Berkshelf
Add `foodcritic` the cookbook linter
Fix foodcritic warnings of FC001 and FC019

```
FC001: Use strings in preference to symbols to access node attributes: ./libraries/provider_td_rubygems.rb:47
FC001: Use strings in preference to symbols to access node attributes: ./libraries/version.rb:5
FC001: Use strings in preference to symbols to access node attributes: ./recipes/default.rb:13
FC001: Use strings in preference to symbols to access node attributes: ./recipes/default.rb:20
FC001: Use strings in preference to symbols to access node attributes: ./recipes/default.rb:95
FC001: Use strings in preference to symbols to access node attributes: ./recipes/default.rb:97
FC001: Use strings in preference to symbols to access node attributes: ./recipes/default.rb:103
FC001: Use strings in preference to symbols to access node attributes: ./templates/default/td-agent.conf.erb:46
FC019: Access node attributes in a consistent manner: ./libraries/provider_td_rubygems.rb:47
FC019: Access node attributes in a consistent manner: ./libraries/version.rb:5
FC019: Access node attributes in a consistent manner: ./recipes/default.rb:13
FC019: Access node attributes in a consistent manner: ./recipes/default.rb:20
FC019: Access node attributes in a consistent manner: ./recipes/default.rb:95
FC019: Access node attributes in a consistent manner: ./recipes/default.rb:97
FC019: Access node attributes in a consistent manner: ./recipes/default.rb:103
```
Fix foodcritic warnings of FC023 and FC043

```
FC023: Prefer conditional attributes: ./recipes/default.rb:88
FC043: Prefer new notification syntax: ./recipes/default.rb:119
```
Lock `serverspec` version

Since `serverspec` v2 is incompatible with v1, tests will not run
without modifications.
Remove a broken test

The `fluent-plugin-datacounter` isn't installed by default. Plus, the
gem version might be changed inside `td-agent` package and it's hard to
expect the version from cookbook tests.