Add InfluxDB::Client::ping

The ping endpoint is useful to check whether the influxdb is alive or not. I'd like to use this method from the [sensu-community-plugins](https://github.com/sensu/sensu-community-plugins) project.

Btw, I had to specify `"rspec", "~> 3.0.0"` in the `influxdb.gemspec` to run the tests. Otherwise I get

```
% bundle exec rake spec                                                                                     ✹ ✭
Your Gemfile lists the gem webmock (>= 0) more than once.
You should probably keep only one of them.
While it's not a problem now, it could cause errors if you change the version of just one of them later.
rake aborted!
TypeError: no implicit conversion of Rake::FileList into String
/home/maoe/src/influxdb-ruby/vendor/bundle/ruby/2.0.0/gems/rspec-core-3.1.1/lib/rspec/core/rake_task.rb:117:in `exist?'
/home/maoe/src/influxdb-ruby/vendor/bundle/ruby/2.0.0/gems/rspec-core-3.1.1/lib/rspec/core/rake_task.rb:117:in `file_inclusion_specification'
/home/maoe/src/influxdb-ruby/vendor/bundle/ruby/2.0.0/gems/rspec-core-3.1.1/lib/rspec/core/rake_task.rb:151:in `spec_command'
/home/maoe/src/influxdb-ruby/vendor/bundle/ruby/2.0.0/gems/rspec-core-3.1.1/lib/rspec/core/rake_task.rb:85:in `run_task'
/home/maoe/src/influxdb-ruby/vendor/bundle/ruby/2.0.0/gems/rspec-core-3.1.1/lib/rspec/core/rake_task.rb:109:in `block (2 levels) in define'
/home/maoe/src/influxdb-ruby/vendor/bundle/ruby/2.0.0/gems/rspec-core-3.1.1/lib/rspec/core/rake_task.rb:107:in `block in define'
Tasks: TOP => spec
(See full trace by running task with --trace)
```