Proposed new overloads to AssertConfigurationIsValid to solve issue #187

* This commit resolves the following issue:
https://github.com/AutoMapper/AutoMapper/issues/187
* Added new overloads of AssertConfigurationIsValid with parameter `bool onlyCheckPubliclySettableProperties`. This parameter allows
properties with private or protected setters to be bypassed during
validation.
* Maintained the original overloads but marked them obsolete.
* Updated all unit tests to use the new overloads.
* Added the following unit test:
When_testing_a_dto_with_non_publicly_settable_properties
* All tests pass.