* expand base read functions to add timeout

* provide readline and readuntil with timeout

* tweak readme

* remove custom serial port

* readuntil continue if no bytes to prevent getting stuck

* fix test copy paste error

print_port_metadata() should not try to print USB attributes unless
the transport mechanism is USB; doing otherwise caused an exception.

…, #53)

The high-level API previously had defined methods of Base.write() and
Base.flush() that behaved quite differently from how Base documents
these functions to behave, and in some cases didn't work at all. By
removing these methods, we instead simply inherit much better working
and more consistent ones from the `IO` supertype instead, making the
high-level API more similar to Julia's file I/O API in Base.

This patch also documents and passes through the low-level functions
sp_drain(), sp_flush() and sp_output_waiting() to the high-level API,
as these serial-port specific functions are what users should probably
be using instead of Base.flush() to ensure UART queues are empty.

Calling loc() in the arguments of handle_error() causes unnecessary
string allocations in the non-error path. The entire loc() function is
unnecessary since Julia's stack traces already provide information on
where an exception occured.