Added aggregate COUNT function

Added aggregate COUNT function per issue #8.

API added:
* `Table.count([alias])` returns ColumnNode
* `Column.count([alias])` returns ColumnNode
* `ColumnNode.as(alias)` returns ColumnNode

Properties added:
* `Column.star` __boolean__ Allows for the field name to be an unquoted `*`
* `ColumnNode.star` __boolean__ Allows for the field name to be an unquoted `*`
* `ColumnNode.aggCount` __boolean__ Triggers the use of the `COUNT()` function.

Tests written in `tests/postgres/aggregate_tests.js`