renamed "Endpoint" to "Context"
rename fine tuning
rename 'endpoint' to 'context'

this mainly affects the aiocoap.Endpoint class (-> aiocoap.Context) and
its create_{client,server}_endpoint (-> create_{client,server}_context)
methods.

as for the reason for the renaming, see aiocoap.Context's docstring.
modernize clientPUT example
documentation enhancements
documentation for tools
sphinxification in optiontypes
installation and development notes