Added byte-wise Shamir secret sharing
Formatted the code and added slice sharing and recovery and a simple test
Added a new interface ByteSharer and implemented it with XOR and Shamir.

This also refactors the original code to use a ByteSharer for sharing and
reassembly of the key.
Added an initial byte to give the share type
Started to add QR codes for output
Converted to base64 before encoding
Separated out encoding from file encryption and changed sharegen to support both
threshold and XOR, as well as Base64 and QR codes.
Fixed the README and set the default threshold to be 0
Fixed golint errors
Fixed style and added more tests for errors
Add a simple test to make sure QR codes work.

This commit adds a test that uses a command-line program to decode PNG QR codes
generated by keyshare. The current implementation depends on the old version of
the zxing cpp source, but any command-line utility that can decode a QR PNG file
will work.
Fix the tests to use better Go style.

This commit shortens the tests for -test.short and uses the bytes package to
check slice equality rather than checking by hand.
Fix a typo in the README.