Provide multiple connection pools

Today resthub comes with a unique bonecp connection pool, configured with default values.

HikariCP seems to be a good and very powerfull alternative and could be proposed to replace BoneCP but it requires Java7 and we do not want to force resthub users to move to Java7.

This PR provides: 
* some extension points to change connection pool
* a default activated (via spring profile) bonecp configuration
* an optional (to activate instead of bonecp) hikaricp configuration
* new configuration allowing to customize any bonecp option (and not only resthub defaults)

**Main changes**

* spring "resthub-pool-bonecp" should now be activated in order to use default provided bonecp configuration
* spring "resthub-pool-hikaricp" could replace the previous one in order to change connection pool
* to use hikariCP, users must add hikaricp maven dependency (optional in resthub) and exclude bonecp :

```xml
         <dependency>
            <groupId>org.resthub</groupId>
            <artifactId>resthub-jpa</artifactId>
            <version>${resthub.spring.stack.version}</version>
			<exclusions>
                <exclusion>
                    <groupId>com.jolbox</groupId>
            <artifactId>bonecp-spring</artifactId>
                </exclusion>
            </exclusions>
        </dependency>
		
		<dependency>
            <groupId>com.zaxxer</groupId>
            <artifactId>HikariCP</artifactId>
            <version>1.2.9</version>
            <optional>true</optional>
        </dependency>
```

* unique `database.properties` configuration file has been splitted into two files `datasource.properties` and `persistence.properties` in order to allow full connection pool options override. Default `datasource.properties` is empty due to resthub defaults (can be ommited).