Djm/endpointfun

[dajmcdon]

1. adds a avail_endpoints() to list all available endpoints
2. adjusts _pkgdown.yml to simplify the reference build
3. removes "Fetch" as the first word in the title of the endpoint man files (it doesn't fetch, and is a bit redundant even if it does)
4. The Meta/ directory should not be under version control.
5. Moved the development.rmd vignette to DEVELOPMENT.md, don't build it.
6. Use && instead of & in covidcast.R (lintr wants this, and it's better for if/else statements)
7. Wrap epirange() in the FluView documentation.
8. Fix S3 method/generic consistency in as.data.frame.xxx() funs (covidcast.R)

closes #154
closes #147
closes #148

[dshemetov]

Thanks Dan!