implementation of the checkbox pagepart
Merge pull request #48 from Kunstmaan/checkboxpagepart

implementation of the checkbox pagepart
Fix invalid ID names
Merge pull request #50 from Kunstmaan/fix_unique_id

Fix invalid ID names
Update README.md
getDefaultOptions() is deprecated since version 2.1 and will be removed in 2.3. Use setDefaultOptions() instead.
fix choice field validation

the first item has value "0" which is empty but not null, so is_null is a better check there
fix typo
Merge pull request #51 from Kunstmaan/fix/choicefieldvalidation

fix choice field validation
fix choice field : required
Merge pull request #52 from Kunstmaan/fix-choice-required

fix choice field : required
fix toString method should always return a string
Merge pull request #53 from Kunstmaan/fix_tostring

fix toString method should always return a string
making form submission list PHP 5.3 compatible
Merge pull request #47 from Kunstmaan/fix_submission_list

View a form submission
Merge branch 'master' into 2.2

* master:
  making form submission list PHP 5.3 compatible
  fix toString method should always return a string
  fix choice field : required
  fix typo
  fix choice field validation
  getDefaultOptions() is deprecated since version 2.1 and will be removed in 2.3. Use setDefaultOptions() instead.
  Update README.md
  Fix invalid ID names
  implementation of the checkbox pagepart

Conflicts:
	Entity/FormSubmissionFieldTypes/BooleanFormSubmissionField.php
	Form/BooleanFormSubmissionType.php