add gnuplot package
fix PATH environment variable
add dependencies to libgd, cairo and libreadline to support more features
Merge pull request #7 from galaxyproject/gnuplot

Add gnuplot package which has been used on the Test Tool Shed.

See https://testtoolshed.g2.bx.psu.edu/view/iuc/package_gnuplot_4_6 with https://toolshed.g2.bx.psu.edu/view/iuc/package_gnuplot_4_6 to follow shortly.