New targets, code improvements and small stuff.

Renames "target" to "engine" throughout the tool; fits better.

Closes issue #16 -- xelatex engine implemented

Addresses issue #20 -- pandoc engine implemented. Open: may have to pass many parameters.

Fixes issue #21 -- check whether engine is installed

Changes code so that engines now have their own classes; cleaner now.

Engine parameters were not actually used; fixed now.