sql/(plan,expression): implement String method to make tree printable

Closes src-d#95
Closes src-d#108

- Implements String method on all sql.Nodes.
- Implements String method on all sql.Expressions.
- Remove Name method from sql.Expressions.
- Only leave Name in expressions that will be Nameable.
- Created TreePrinter to print trees with a Node and Children.

Signed-off-by: Miguel Molina <miguel@erizocosmi.co>