[#1] Added micro_validator test environment and tests.
[#1] Added Errors#all
[#1] Variable arguments in validate method.
[#1] Fixed micro_validator tests.
[#1] Added instance method Errors#pick_one.
[#1] Renamed Errors#pick_one to Errors#field.
Merge pull request #2 from krzyzak/1_add_errors_and_tests

[#1] Added test environment, all() and pick_one() methods.