analysis of message length distribution over conversation
add file_utils
reorg

cache prcessing in pickle files
make a function generate_conversation_message_lengths
partial progress on conversation functions
add helper functions to operate on messages at the conversation level
fully use sdl_utils for message length generation
generate boxplot subsets for {teen,counselor}_{good,bad}_message_lengths
differentiate between good and unrated conversations
create separate plots for unrated conversations
remove unused function get_conversation_num_chars_per_message
add variable containing absolute path of repo
use constants.py for file locations
Merge branch 'sdl_analysis'