Add version to plugins

Plugin developpers can now add a version number to their es-plugin.properties file:

```properties
plugin=org.elasticsearch.test.integration.nodesinfo.TestPlugin
version=0.0.7-SNAPSHOT
```

Also, for site plugins, it's recommended to add a `es-plugin.properties` file in root site directory with `description` and `version` properties:

```properties
description=This is a description for a dummy test site plugin.
version=0.0.7-BOND-SITE
```

When running Nodes Info API, you will get information on versions:

```sh
$ curl 'http://localhost:9200/_nodes?plugin=true&pretty'
```

```javascript
{
  "ok" : true,
  "cluster_name" : "test-cluster-MacBook-Air-de-David.local",
  "nodes" : {
    "RHMsToxiRcCXwHiS6mEaFw" : {
      "name" : "node2",
      "transport_address" : "inet[/192.168.0.15:9301]",
      "hostname" : "MacBook-Air-de-David.local",
      "version" : "0.90.0.Beta2-SNAPSHOT",
      "http_address" : "inet[/192.168.0.15:9201]",
      "plugins" : [ {
        "name" : "dummy",
        "version" : "0.0.7-BOND-SITE",
        "description" : "This is a description for a dummy test site plugin.",
        "url" : "/_plugin/dummy/",
        "site" : true,
        "jvm" : false
      } ]
    },
    "IKiUOo-LSCq1Km1GUhBwPg" : {
      "name" : "node3",
      "transport_address" : "inet[/192.168.0.15:9302]",
      "hostname" : "MacBook-Air-de-David.local",
      "version" : "0.90.0.Beta2-SNAPSHOT",
      "http_address" : "inet[/192.168.0.15:9202]",
      "plugins" : [ {
        "name" : "test-plugin",
        "version" : "0.0.7-SNAPSHOT",
        "description" : "test-plugin description",
        "site" : false,
        "jvm" : true
      } ]
    },
    "H64dcSF2R_GNWh6XRCYZJA" : {
      "name" : "node1",
      "transport_address" : "inet[/192.168.0.15:9300]",
      "hostname" : "MacBook-Air-de-David.local",
      "version" : "0.90.0.Beta2-SNAPSHOT",
      "http_address" : "inet[/192.168.0.15:9200]",
      "plugins" : [ ]
    },
    "mGEZcYl8Tye0Rm5AACBhPA" : {
      "name" : "node4",
      "transport_address" : "inet[/192.168.0.15:9303]",
      "hostname" : "MacBook-Air-de-David.local",
      "version" : "0.90.0.Beta2-SNAPSHOT",
      "http_address" : "inet[/192.168.0.15:9203]",
      "plugins" : [ {
        "name" : "test-plugin",
        "version" : "0.0.7-SNAPSHOT",
        "description" : "test-plugin description",
        "site" : false,
        "jvm" : true
      }, {
        "name" : "test-no-version-plugin",
        "version" : "NA",
        "description" : "test-no-version-plugin description",
        "site" : false,
        "jvm" : true
      }, {
        "name" : "dummy",
        "version" : "NA",
        "description" : "No description found for dummy.",
        "url" : "/_plugin/dummy/",
        "site" : true,
        "jvm" : false
      } ]
    }
  }
}
```

Relative to #2668.
Closes #2784.