New and improved Documenter based documentation

[AsafManela]

• Added docstrings to most exported functions and updated existing ones
• Replaced readthedocs-based documentation with Documenter.jl-based ones
• New docs provide examples for how fit a RegularizationPath or a RegularizedModel to a dataframe

[coveralls]

Pull Request Test Coverage Report for Build 168

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 10 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-1.1%) to 87.403%

---

Files with Coverage Reduction	New Missed Lines	%
src/segselect.jl	3	71.7%
src/Lasso.jl	3	63.72%
src/coordinate_descent.jl	4	54.21%
<!--	Total:	10		-->

Totals
Change from base Build 167:	-1.1%
Covered Lines:	562
Relevant Lines:	643

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 169

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 10 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-1.1%) to 87.403%

---

Files with Coverage Reduction	New Missed Lines	%
src/segselect.jl	3	71.7%
src/Lasso.jl	3	63.72%
src/coordinate_descent.jl	4	54.21%
<!--	Total:	10		-->

Totals
Change from base Build 167:	-1.1%
Covered Lines:	562
Relevant Lines:	643

---

💛 - Coveralls