Add "errors_on" support for ActiveModel validation
Change Input#share_group to be a Symbol
Add Input.by_share_group

Returns a hash, each key being the name of a share group, and each value
an array of inputs belonging to the group.
Assert that share_group may not be zero-length

The share_group value must be either nil (omitted completely), or have a
non-zero length value (i.e. a string or symbol containing at least one
character).
Assert that share_groups in presets sum to 100

Ref quintel/etengine#465
Add the correct license (MIT)
Update development dependencies