Add MIT License
Removed travis support
Fixed some vet issues
Validate too many options error
Better error message on invalid option value
Handle encoding MediaType
Test large option types
nil makes a better empty slice
Remove exported options slice, replace with methods

This was a problem with vet, but muhc of this kind of thing has to
happen in apps anyway, so this may very well be easier to use.
Make the examples compile again.
option doesn't need to be exported
More consistent and useful COAPCode (with String())
String() for COAPType
Fix weird looking error return
Take message pointers in handlers
Mechanism for picking up Options from a Message
Reusable options -> []string extractor
Reuse server buf

Instead of generating a new huge buffer for every request, just
duplicate the relevant part from the read.
encodeMessage(Message) -> Message.encode()
Extraneous line