Add @Unique annotation

This annotation will specify that a column is to be unique and as such
can be used as a candidate key. As written, this annotation can be
combined with any other annotations.

There is a provided test which fails as written since it relies on an
actual database.
Fixes #10
Merge branch 'unique' into v2

Conflicts:
	library/src/main/java/se/emilsjolander/sprinkles/ColumnField.java
	library/src/main/java/se/emilsjolander/sprinkles/DbOpenHelper.java
	library/src/main/java/se/emilsjolander/sprinkles/Migration.java
	library/src/main/java/se/emilsjolander/sprinkles/Utils.java
	library/src/main/java/se/emilsjolander/sprinkles/annotations/ConflictClause.java
	library/src/main/java/se/emilsjolander/sprinkles/annotations/Unique.java