Implement tail -n 20 in pure Ruby

[mattbrictson]

I think full-blown tail implementation is overkill, so I went with a solution that should work 99% of the time (as long as the line lengths are not extremely long):

• Read the last 8 KB of the file
• Split that into lines
• Return the last 20 of those lines

If for some reason the last 8 KB has fewer than 20 lines, it will not seek backwards to create a bigger buffer; instead it will just return the lines it already has.

Closes #31.

[mattbrictson]

@robd I think you were the one who originally suggested we replace tail -n 20 with a pure Ruby equivalent. I've implemented something like that in this PR. Do you mind taking a quick look?

[robd]

@mattbrictson sorry for the delay here. Agree that full blown tail impl is overkill. This looks good to me.