Table Creation DSL

[groue]

This PR closes #83.

It contains:

• A DSL for creating and altering tables and indexes
• Support for the LENGTH SQLite built-in function (it helps documenting CHECK constraints)
• New SQLCollation enum replaces collation names everywhere in the API
• Extensive testing and documentation
• Unrelated: renaming of PrimaryKey into PrimaryKeyInfo, for consistency with ColumnInfo and IndexInfo.

Many thanks, @cfilipov, for your intense and great contributions. This would have simply not existed without you.

[groue]

@cfilipov Support for table creation DSL has landed in v0.78.0, now merged into the Swift3 branch (documentation).

Implementation was rather straightforward, since we two have been discussing the subject in depth: https://github.com/groue/GRDB.swift/blob/Swift3/GRDB/QueryInterface/SQLTableBuilder.swift

You'll see that:

• SQLColumn has been renamed Column, as you have suggested 😄
• The ASC/DESC in INTEGER PRIMARY KEY ASC is not supported: I simply did not understand the purpose/usefulness of these modifiers.
• Index on collated columns are not supported: CREATE INDEX foo ON bar(baz COLLATE NOCASE)
• Default values for columns are defined with a DatabaseValueConvertible value, not an expression. This is because SQLite, in practice, only accepts literals and expressions that evaluate to a constant.

There is always room for improvement: I hope that you'll provide feedback after you have played with this new API a little.

[cfilipov]

@groue This looks wonderful! I've pulled the latest Swift3 branch and started playing with the lib. Switching to GRDB has been great, I'm really happy with the quality of this lib.