Added ICAP SEF capability (#3)

Using dplyr functions
Updated .gitignore
Refactored BSEF data

- moved bits of get_bsef_eod_data() function into two new functions,
download_bsef_data() and format_bsef_data() per ICAP equivalents.
- get_bsef_eod_data() renamed to get_bsef_data() per get_icap_data()
- start_date argument in get_bsef_data() renamed to date per
get_icap_data()
- making use of dplyr functions
Appended `_single` to download_bsef_*()

Will vectorise on asset_class argument to make it easier to get all
data for a given date.
get_bsef_data() not iterates across asset classes

Also oxygenised.
Added messages for BSEF functions
Documented get_cap_data()

Also moved function to start of R file