Fixes failed catching of exception.
Removes catching of exception in play_index, catches it in play() instead.
Removed parens.
Merge pull request #19 from petteraas/play_index_exception

Play index exception
Adds remove 'index' command.
Added extra space between functions.
Moved 'in queue' check out of play_index to separate function is_index_in_queue
Added is_index_in_queue check in remove_from_queue
Explained indexing translation.
Catch ValueError in process_cmd.

Added helper function get_queue_length

is_index_in_queue takes queue_length parameter and always returns boolean.

Added remove_index_from_queue
Fixed
C:299, 0: Wrong hanging indentation.
index,
   ^ (bad-continuation)
C:300, 0: Wrong hanging indentation.
queue_length)
^ (bad-continuation)