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Merge pull request #128 from open-app/markdown-bucket-descriptions

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default contribution amount to 0, fix #130
when contribution.amount is zero, remove contribution instead of save.

fix #129
delete id from removed model
split round-modal into separate module
re-use round modal as a stop-gap round editor, add edit button to group rounds page
rename round modal newRound form to roundForm
add inputs for start and end times, merge with date inputs
add sane defaults to new round modal
return saved object from RestfulStore#save when creating
use ui.bootstrap.datetimepicker in round modal based on @robguthrie's recommendation
add ng-required and input name to datetimepickers
styled round edit button
fix minor isadmin bug on edit button