README: usage examples are for awesome version 3.4
wifi: add support for /usr/bin binary path

Years ago iwconfig started its life in /sbin. Then some distributions
moved it to /usr/bin. Then in 2012 some projects started pushing for
deprecation of /usr/sbin and merging everything into /usr/bin.

We now search paths including /usr/bin for the iwconfig binary. This
is because sbin paths are not usually in non privileged user PATH, so
we help io.popen locate the binary on the file-system.

We do this search in the following way:
  - default to basename only, 'iwconfig' for the iwconfig variable
  - search paths one by one once and redeclare variable iwconfig with
    full path if binary is found in any
  - avoid searching paths on next execution if iwconfig variable
    already contains a path
Next release, tag 2.1.1
Merge branch 'master' of http://git.sysphere.org/vicious