Cover blockWhenFull in README; update xUnit; minor tidying

[bartelink]

This PR is a grab-bag of cleanup work, which arose in preparation work for #29 :-

• document blockWhenFull (added in Fulfills #20 - add option to block when the queue is full, instead of dropping events #21)
• update xUnit to 2.3.1 (so it runs correctly in VS2017) (includes addressing warnings emitted by its validator)
• minor reformatting

[bartelink]

(This got hit by a timing sensitivity in an existing test; will shore it up when I get to office; eta ~1h)

[bartelink]

@nblumhardt Should be good to go now; same commits are at start of #29 so merging should make that clearer

[nblumhardt]

Awesome, thanks @bartelink