Read performance updates

* increase default buffer size, add `bufsize` argument to `Reader`
* write decompress data directly to `IOBuffer` (using an unexported but likely stable for the near-future interface--see https://github.com/JuliaLang/julia/pull/4484)
* Added optimized `readuntil()` (takes advantage of `IOBuffer` use)
* update to `eof()` to short-circuit buffer filling

With these changes,

* `readline` is ~5-6x faster
* larger buffer sizes offer modest speed improvements

Before these improvements, it took ~13 seconds to read all lines in a 279Mb text file (56Mb compressed).  After, it takes ~2.3 seconds with a 4Kb buffer, <2.1 seconds with a 128Kb buffer.  For comparison, gzip takes ~2.0 seconds to decompress the file.