changed indeterminate behavior

Hello,

i'm currently working on a project, where i'm relying on the indeterminate feature of checkboxes.

I like the way of displaying the indeterminate state with bootstrap-switch, but when a switch is in indeterminate state, the behavior is not way it's supposed to be.

When a checkbox is in indeterminate state, and the user clicks (or changes the value in any other way), the checkbox has to leave it's indeterminate state and from then on behaves like a normal checkbox.

Your implementation disables changing the value of the checkbox, when it's indeterminate.

My pull request includes the changes, to make bootstrap-switch behave like it's expected.

Some more words here:
http://www.wikiwand.com/en/Checkbox#/Tri-state_checkbox
http://stackoverflow.com/questions/10099158/how-to-deal-with-browser-differences-with-indeterminate-checkbox

I hope you can use it.

Cheers