Large refactor ready for version v2.0.0

[geraintpalmer]

• Drop support for Python 2.7, Python 3.4
• Update networkx and pyyaml requirements
• Refactor time so that Simulation has current_time attribute
• Change Transition_matrices keyword to routing
• routing can take a process-based routing function
• Refactor distributions to be objects: ['Exponential', 0.5] ->ciw.dists.Exponential(0.5)
• Distribution objects can be manipulated with +, -, * and /
• All keywords lower case to conform to Pep8
• deadlock_detector keyword takes object, not string
• tracker keyword takes object, not string
• Add tests and docs to show how objects can be used for state-dependent distributions
• All user facing api now takes float('inf' not 'Inf, expect for .yml files

[coveralls]

Coverage remained the same at 100.0% when pulling 421a858a13c1e914c93a4412f1be265966b34dea on next-release into 4f677f646681e68af05d2e8d6d0e49783917f1f2 on master.