New method in solution for reporting metabolite balance details.
Added padding to Solution print methods.
Improvements in Solution printing format.
Merge pull request #14 from cdanielmachado/marta-branch

Marta branch
Show reaction equations in solution output.
Revert 98de0df..d4fc7ee

This rolls back to commit 98de0df326d6dab4c481e3ecd8d4ef6897496971.
Fixes to show_metabolite_balance method.
gapFind/Fill, glpk (lazy) interfaces, test files

The gapFind/gapFill methods were added. Also the glpk interfaces with
and without lazy loading (note that the latter is faster, in some cases
even faster than gurobi).
Unit tests for the solvers, gapFill, and gapFind were added, as well as
the necessary files in the models folder.