added $pmux cell translation
fixed memory next issue, when same memory is written in different case statement
fixed reduce_xnor, logic_not bug translation bug
Merge branch 'master' of https://github.com/cliffordwolf/yosys into btor
added case for memwr cell that is used in muxes (same cell is used more than one time)
corrected bug for xnor and logic_not
added pmux cell translation

Conflicts:
	backends/btor/btor.cc
Merge pull request #39 from ahmedirfan1983/master

merged with current mas.ter branch + features added + bug fixes