Adding database admin support in influxdb_user resource

## Adding db admin feature
ref issue #31 :

Adding a new ```dbadmin``` attribute in ```influxdb_user```.
* This attribute is optional (default is empty)
* It takes an array of database name
* it creates specified user into db and gives him admin rights on all the db listed in ```dbadmin```
* it can be used with current ```databases``` attribute.
* ```databases``` attribute is now optional (default is empty) : if no databases are specified either in db or dbadmin, the user will not be created (it's not an error)

## Example

**Recipe :**
```
influxdb_user 'nothing' do
  password 'nothing'
end

influxdb_user 'user' do
  password 'user'
  databases ['metrics']
end

influxdb_user 'admin' do
  password 'admin'
  dbadmin ['metrics']
end

influxdb_user 'both' do
  password 'both'
  databases ['metrics']
  dbadmin ['metrics']
end
```

**Log**
```
  * influxdb_user[nothing] action create
  Recipe: <Dynamically Defined Resource>
    * chef_gem[influxdb] action install (up to date)
     (up to date)

  * influxdb_user[user] action create
  Recipe: <Dynamically Defined Resource>
    * chef_gem[influxdb] action install (up to date)
     (up to date)

  * influxdb_user[admin] action create
  Recipe: <Dynamically Defined Resource>
    * chef_gem[influxdb] action install (up to date)
     (up to date)

  * influxdb_user[both] action create
  Recipe: <Dynamically Defined Resource>
    * chef_gem[influxdb] action install (up to date)
     (up to date)
```

**Result**
![selection_010](https://cloud.githubusercontent.com/assets/1901955/4578488/35d85e52-4fc5-11e4-8ca8-565c175e89cb.png)