use EXTRA_TEXT when receiving text
throw an exception on null data. display a toast.
Revert "throw an exception on null data. display a toast."

This reverts commit 12ce60a62814d98625a91f4d44049b9f02694de3.
use .equals for string compare
encode text and subject
handle the custom mime type at the right place
handle the mime type at the receiver
restore the text/plain mime type
updated check headers script
updated readme paths
formatted README slightly
clarified the output of checkheaders
added copyright headers
fixed a check headers bug
merged MainActivity
merge master with ticket-22