added support for AliasExists with unit tests
Merge branch 'develop' into feature/alias-exists

Conflicts:
	src/Nest/IElasticClient.cs
	src/Nest/Nest.csproj
	src/Tests/Nest.Tests.Unit/BigBadUrlUnitTests.cs
added integration tests for AliasExists() and convenience extensions
Merge pull request #851 from elasticsearch/feature/alias-exists

AliasExist() support