pmt_io: adding string conversion for uniform vectors

Sadly, this needed expansion of unv_template, otherwise I'd have to
hand-write gengen-style code in pmt_io.cc

partly in response to http://lists.gnu.org/archive/html/discuss-gnuradio/2014-04/msg00168.html , partly because I had that same problem and used massively ugly workarounds.

Downside of patch: 
- introduction of string functionality in the individual uniform vector classes
- dependency on boost/lexical_cast (is dependency of gnuradio-runtime, anyways)