Add support for specifying a different database name for fields

[pikaju]

Adds custom column names with one of the following syntax options:

#[column("name")]
#[column(type = "type")]
#[column("name", type = "type")]
#[column(type = "type", "name")]

Removes the #[db(type)] attribute.

You can also either specify a varchar type like before:

#[column(type = varchar(5))]

or a custom type with a raw string:

#[column(type = "jsonb")]

Also adds two tests to check if the column was really renamed.

Closes #153

[carllerche]

We probably want this to be String to allow runtime specification of custom types. I don't think allocations will be an issue.

[pikaju]

Okay, I changed it to String

[carllerche]

Left a comment inline.

[carllerche]

I'm OK merging the PR with this, but as a follow up, I don't think we need to track app_name here. app_name is the app-level field name. db::Column is the storage name. If there is something I am missing, let me know.