Feature/get field mapping

This fixes #505 by implementing the [get field mapping API](http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/indices-get-field-mapping.html#indices-get-field-mapping).

See the integration tests for usage:

https://github.com/elasticsearch/elasticsearch-net/blob/feature/get-field-mapping/src/Tests/Nest.Tests.Integration/Core/GetFieldMapping/GetFieldMappingTests.cs