v0.14.2: Documenter (#640)

* Renamed doc/ to docs/.

* Updated gitignore with new docs directory (and docs/site).

* Added documenter builds to travis.

* Updated link in README and added latest badge.

* Started restructuring docs directory.

* Renamed all the .rst files to .md.

* Added JuliaStats logo.

* First pass converting the restructured text to markdown.

* Migrated function docs to docstrings for the appropriate types. NOTE: This often required making some default methods which fail if dispatched to.
* Added some missing references when possible.
* Added a few (mostly empty) docstrings on a few undocumented, but exported distributions.
* Fixed a few equation formatting issues (e.g., `FDist`).

* Minor fixes, but some of them are kind of hacks around the documentation system.

* Turned docstring methods into empty methods that won't be dispatched to.

This allows us to have custom docstrings that won't introduce new methods that could change the behaviour.
Reverted changes to testutils.jl

* Fixed bad references in some univariate docstrings.

* A few formatting fixes.

* Proper escaping of all the latex equations to support 0.7.

Looks like the Base.Markdown.MD `doc""" ... """` syntax doesn't play well with escaping latex equations.
The result is poorly formatted equations in documenter, but using normal docstrings works fine.

* Fix a few more missing escapes

* Convert `NoncentralChisq` to normal docstring so that Documenter can render the equations properly.