Add throttling to snaphost and restore operations

Closes #4855
fixed date_/histogram aggregation documentation - added documentation for the `min_doc_count` setting

Closes #4944
Fix javadocs for DeleteResponse.isFound()
fixed a bug where when executing on a single shard and sorting terms agg based on sub metric agg, the order (asc/desc) is not respected
 - fixed tests for terms order

Closes #4951
REST Get Field Mapping API: Fix NPE if field not existent

When fixing #4738, a small issue leaked into the implementation.
The equals check in the RestAction only applied when the master node
returned the rest request, otherwise the object equality would not hold
due to being transferred over the wire and being deserialized into
another object (from and an equality point of view) than the
FieldMappingMetaData.NULL object - this could result in serialization
exceptions as an empty length bytes reference is used in toXContent.
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.
Add _cat/plugins endpoint

If we want to have a full picture of versions running in a cluster, we need to add a `_cat/plugins` endpoint.

Response could look like:

```sh
% curl es2:9200/_cat/plugins?v
node component                        version
es1  jvm                              1.6.0_65-b14
es1  elasticsearch                    0.90.10
es1  elasticsearch-mapper-attachment  1.7.0
es1  elasticsearch-lang-js            1.7.0
es1  elasticsearch-analysis-smartcn   1.9.0
es2  jvm                              1.6.0_65-b14
es2  elasticsearch                    1.0.0.RC1
es2  elasticsearch-mapper-attachment  2.0.0.RC1
es2  elasticsearch-lang-js            2.0.0.RC1
es2  elasticsearch-analysis-smartcn   2.0.0.RC1
```

Closes #4824.