Added indices aliases exists api.

Added indices aliases exists api that allows to check to existence of an index alias. This api redirects to the master to check for the existence of one or multiple index aliases.

Possible options:
* `index` - The index name to check index aliases for. Partially names are supported via wildcards, also multiple index names can be specified separated with a comma. Also the alias name for an index can be used.
* `alias` - The name of alias to check the existence for. Like the index option, this option supports wildcards and the option the specify multiple alias names separated by a comma. This is a required option.
* `ignore_indices` - What to do is an specified index name doesn't exist. If set to `missing` then those indices are ignored.

The rest head endpoint is: `/{index}/_alias/{alias}`

Examples:
Check existence for any aliases with the name 2013 in any index:
```
curl -XHEAD 'localhost:9200/_alias/2013
```
Check existence for any aliases that start with 2013_01 in any index
```
curl -XHEAD 'localhost:9200/_alias/2013_01*
```
Check existence for any aliases in the users index.
```
curl -XHEAD 'localhost:9200/users/_alias/*
```

Closes #3100