Merge pull request #29 from ryotarai/source-in-gemfile

Use https://rubygems.org as the rubygems source.
Support log_level option
Bump version 0.7.2
Update .travis.yml
Adding a couple of links for Fluentd
s/Fluent/Fluentd in gemspec
Update travis-ci setting
Sanitize keys which contain "." or start with "$".
Add replace_dot_in_key_with and replace_dollar_in_key_with to config
params.
Use normal method instead of extending HashHelper module.