implemented changes to ask the user before enabling bluetooth.

removed auto enable of bluetooth from ConnectionService onCreate

added wrapper class to ask the user, enable bluetooth, and then run supplied
code

implemented use of wrapper on "Join" button (connectfragment), "Add users"
button, and "Switch networks" button (networkfragment)
added comment to explain the noop cancel button listener
Merge pull request #308 from theJenix/ask_enable_bt

implemented changes to ask the user before enabling bluetooth.