feature start: Recognize and skip release stories

The plugin was completely ignoring that there may be some stories of the release
type, which were supposed to be skipped during feature start like chores are.

Change-Id: I289c67293b3c3420adf0d2431f3a924b1acf4347
Signed-off-by: Ondrej Kupka <ondra.cap@gmail.com>
Finished feature/ 52543807/feature-start-fails-on-release-story-type

Change-Id: Ic6967905a2d352d15f846f7ddf522a3a101a21be
PT-Story-URL: http://www.pivotaltracker.com/story/show/52543807
RB-Review-Request-URL: https://dev.salsitasoft.com/rb/r/4239/