Add support for using index algorithm

[lillo42]

Add support for using index algorithm:

• BTree
• Hash
• Gist
• Spgist
• Gin
• Brin

[jzabroski]

This looks great, but I don't see (and was expecting to see) a PostgresExtensions with a signature that let's you pass in the Enum? Am I just being blind looking at the PR on GitHub and I don't see that interface?

Basically, I see .UsingBtreeAlgorithm(), etc, but I dont see .Using(Algorithm.Btree). The rationale for letting people pass in an enum is it is easier to parameterize. One use case would be to demo the differences in performances between two index methodologies. Another use case would be to allow people to use FluentMigrator as a dynamic tooling API to generate scripts from within a GUI.

[jzabroski]

This pr is fine. I will merge this weekend