Create datatable-joins.Rmd for Joins vignette

[zeomal]

Primarily aims to close #2181.

First draft of the proposed new vignette for data.table's join functionality. Covers the following:

• Equi-joins
• Aggregate by join (.EACHI)
• Join by reference
• Update by join (using fcase)
• Update multiple columns at once (mget and get and the i. notation, wrapping Docs: explain and document the i.col notation for joins #1038, and partially Vignette for programmatic data.table (eval, get, etc) #4247)
• Traditional joins (left, right, inner, full, anti, semi) using data.table and merge
• setkey vs. on performance comparison and recommendataion (wraps up [Documentation] Use of the on= argument for joins #1623, on performing slower than double setkey #1232 and [R-Forge #2302] Add mnel's merge join syntax to FAQ (with timings) #515)
• allow.cartesian rationale covered
• Covers unexpected subsetting results #2396 - is implicitly covered, is an explicit one required?

Pending coverage:

• overlapping, rolling and non-equi joins
• set operators
• mult

Waiting for implementation:

• nomatch for other values (Expand nomatch to accept any value (control fills, rolls, omits during joins) #857)
• mergelist ([R-Forge #2461] Faster version of Reduce(merge, list(DT1,DT2,DT3,...)) called mergelist (a la rbindlist) #599)