Install below `/usr/local` by default.  Fixes #78
Move `udev` rules file to `libambit`.  Fixes #80

The rules file has been renamed to follow convention.  Many rules files
are named after the library or package that provides it.  Selecting the
numeric prefix is best left to package maintainers.  For local installs
no prefix is necessary and the file will be processed last.
Install `udev` rules in the local administration directory

Package maintainers would install below `/lib/udev/` and we do not
want to clobber their work (see also #78).
Support `DESTDIR` installs

Absolute paths to a `cmake` `install` don't get the same support for
this as relative installation paths, IIUC.

`DESTDIR` installs make it easy to test installs and also makes it a
good deal easier for distriubtions to put their packages together.
Refactor the `udev` rules

We now skip the whole bunch unless `udev` detected the addition of a
Suunto device.  For all devices that `libambit` knows about, set an
environment variable and use that to trigger device access privilege
modifications.