AEMO data packages; enhancements; bug fixes

- NEW: AEMO data set available from Dec 1998 to May 2014
- UPDATED: Added TAS & SNOWY regions for time periods for which these
are available.
- FIXED: Process collated data frames
- FIXED: AEMO url stub fixed
- FIXED: roxygen directives & roxygenised
Improved user abilities; docs expanded

- UPDATED: user can now specify the path to which AEMO CSV files are
downloaded.
- UPDATED: data sets fields of type string are not coerced to factors.
- UPDATED: documentation expanded and clean_up_aemo_data_file() is
exported.
Fixed some check() issues

- Added examples
- Import lubridate's ymd_hms()
- Fixed check() NOTES & WARNINGS
Fixed some doc bugs
Subsetted AEMO data set to reduce size
Updated README
Added test case
Added travis CI