Refactor filepicker_field

* allow to define arbitrary options
* update option naming to current API
Add secret key as config variable
Add Policy class
Secure file picker using policy and signature
Fix small issues
Fix signature
Add initializer to Policy class
Set expiry to default value when not present
Add support for new path option
Add writer for default_expiry
Merge branch 'feature/issue-22'

Conflicts:
	lib/filepicker/rails/form_builder.rb