Now handle missing data / add demos

• revised plot.twoway() to use and default to na.rm=FALSE
• revised plot.twoway.fit() to use and default to na.rm=FALSE
• revised meanfit() to use and default to na.rm=FALSE
• revised twoway.default() to pass ... (hence na.rm) to meanfit
• added demo/ directory
• added demo/example-na.r to illustrate use of na.rm
• added demo/00Index
• Turn on travis CI