frankv gains by= argument, updated TODO in tests.
Added 'dense' and 'runlengths' rank types.
Hook frankv with 'dense' rank.
Merge branch 'Feature-760'

Conflicts:
	R/setkey.R
Added instructions to follow for Bug/FRs/PRs
Added "squashing commits" to PR.
Added links for PR/squash committing PRs
Updated link, and restructured for clarity.
Update Contributing.md
Updated README
Merge branch 'master' of https://github.com/Rdatatable/data.table
Updated README
Update Contributing.md
Remove 'order' argument from frankv
Booked shallow, na.omit in frankv. data.table tests to be added.
Merge branch 'master' into 1.9.8
dcast takes multiple 'value.vars' and fun.agg. Closes #739, #716.
Collect all dcast tests in one place.
Collect all 'melt' tests together.
Added tests for new dcast features.