v2.0.0

As detailed in "Wang H et al. (2018) bioRxiv doi:10.1101/321067"

Main improvements in this release:

• fix generation of folders by exportForGit
• fix dealing with grRules in model I/O
• rename addGenes to addGenesRaven for full cobra-compatibility
• remove version.txt from devel branch, only master branch has numbered releases