Remove implementation of no_constraints in main_adapter

This pull request removes the implementation of `no_constraints` in `main_adapter`.  This is potentially a non-backwards compatible change but the functionality is not being tested for as is (the tests were skipped) so it seems prudent to remove the implementation as well as the tests.  The other changes are refactorings that do not affect the class' behavior.

  * Remove skipped tests in `test/ddl_test.rb` that test now unsupported
    (as of PostGIS 2.0) `no_constraints`
  * Remove implementation of `no_constraints` in `main_adapter`
  * Create private `set_dimensions` method in `main_adapter`
  * Refactor `main_adapter#add_column` method to use private
    `add_spatial_column` method

I'm using this gem on a project and am getting up to speed so I can contribute.  Thanks.