Added support for function validation

Added a new validation that allows to call function in order to
validate. data-fnvalidation=‘function_name’.
Multiple functions validation

Added the option to define functions via the initial options
Added multiple function validation via data-custom
Solved bug with error messages

Updated related test
Added new option 'inputSelector'

Added option ‘inputSelector’ to give the option to choose which
elements validate. This is useful in cases that we only wanna validate
visible elements. {inputSelector: ‘:visible’}.
By default the value is ‘:enabled’
Merge branch 'valid-selector'

Conflicts:
	js/tests/unit/validator.js