Better docs, tests, and model selection

• Allows to specify path segment selector using the select keyword argument to fit(), e.g., select=:BIC
• Updated docs
• Removed julia 0.7 from CI
• Changed unstable tests of degenerate hurdle cases

Upgraded syntax for julia 0.7 and above

v1.0.0

merged 0.7+ syntax upgrade

Version that works with julia v0.6 but not v0.7 and above

• better handling of counts matrices with Int eltype
• better handling of categorical arrays

Cleaned up code and improved test coverage

• Improved tests
• Removed cv functionality which was somewhat application specific