Minor enhancements and bug fixed

New Features:

1. inputType property is added into QuestionText. #141
2. onServerValidateQuestions survey callback is added. Use it to validate question values on the current page on the server, #137.
3. Add/Remove properties run-time: #135

Bug fixes:

1. Question numbers were not update correctly on show/hide questions on the same page: #121
2. Survey doesn't update correctly navigation buttons and progress if next page(s) become(s) change visibility: #123
3. Using a checkbox with react throws a warning about an uncontrolled input becoming controlled: #131. Found and fixed by @jdalegonzalez
4. value for Dropdown question with choicesByUrl isn't returned in survey.data: #138
5. required columns for matrixdropdown don't display error message in knockout version: #142