reconhub/aweek: aweek version 0.2.0

This release fixes a test, allows subsetting and concatenation, and enforces stricter guidelines on character inputs.

• Subsetting and concatenating methods added to the aweek class (see #1)
• Documentation divided into smaller chunks
• as.POSIXlt() bug where tz was not being passed was fixed.
• date2week(): an error is now issued if users specify non-ISO 8601 dates OR don't specify a format option. (found: @scottyaz, #2)
• Best practices added to vignette
• Fix test that would fail every seven days on CRAN (see #5)