Clarify and check UTF-8 encoding of device strings
Substitute "magic" numbers
Replace `ambit_device_info_t *` use with a C++ `DeviceInfo` object

This eases working with the C character arrays from `libambit` as we
can now map back and forth between the assumptions of that library,
UTF-8 encoding for example, and the `openambit` code in `DeviceInfo`
constructors and assignment operators.
Fold support status propagation into `DeviceInfo`
Make `ambit_device_info_t` use null terminated strings

This also fixes up confusion about the various types of names.  There
is a `name` for display, there is a `model` for the clock's nick name
and then there is the HID device's `product_string`.
Fix logical scoping of `#endif` and C89 variable declaration scope
Prioritize local version when resolving libambit header file and shared object. Fixes #77
Make default settings coherent in code and settings dialog
Make `wchar_t` and byte string conversions a utility

The `utf8memconv()` function now takes an extra argument to indicate
the encoding of the string that is passed in.
Forward `wchar_t` conversion to `utf8memconv()`
Make `activity_name` a UTF-8 encoded string.  Re #70

The changeset tries to make sure that allocated memory is released
properly (based completely on code review).
Make the example code buildable again
Remove unused-but-set-variables
Fix message to match failure
Conditionally build the example code and wireshark dissector

To activate, `BUILD_EXTRAS=1 ./build.sh`.
Pass `./build.sh` command-line arguments on to `cmake`

This makes it easier to do, for example, a debug type build.  Simply
add `-DCMAKE_BUILD_TYPE=Debug` to the `./build.sh` command-line and
you're good to go.
Drop install prefix for the extras

They aren't really supposed to be installed (at this point at least)
and a value of `/usr` gets in the way of package management tools.
Sync the README to match the current code base
Don't require a C++ compiler for libambit and the example
Clarify shell flavour expectations

The default `/bin/sh` on Debian (`dash`) does not have `[`, only
`test`.