Scripted metric aggregation support

Closes #1063
Merge branch 'develop' into feature/aggs-scriptedmetric
fixed double implementation of language in develop and this branch causing the tests to fail
Made .Value propery internal and renamed to _Value, and renamed ValueAs<>() to Value<>()
added some xmldocs to Value<>()
Merge pull request #1135 from elasticsearch/feature/aggs-scriptedmetric

Scripted metric aggregation support