Add LZ4 compression as an alternative to LZMA

See also

  - http://code.google.com/p/lz4/
  - http://en.wikipedia.org/wiki/LZ4_%28compression_algorithm%29

LZ4 is an algorithm belonging to the LZ77 family. It's fast and it
satisfies our requirements regarding lack of checksumming and the way small
changes at the end of the input affect the output.

This patch adds LZ4 to the libskr library. The skr binary and
benchmark/measure are still hardcoded to use LZMA, though.

LZ4 allows Skrót to (de)compresses significantly faster compared to LZMA.
Results of benchmarks are following.

  > summary(read.csv("people.csv"))
      original        encoded          ratio            t_enc
   Min.   :117.0   Min.   :62.00   Min.   :0.5116   Min.   :0.4075
   1st Qu.:127.0   1st Qu.:72.00   1st Qu.:0.5623   1st Qu.:0.4207
   Median :133.0   Median :78.00   Median :0.5854   Median :0.4485
   Mean   :132.8   Mean   :77.81   Mean   :0.5851   Mean   :0.4616
   3rd Qu.:138.0   3rd Qu.:82.25   3rd Qu.:0.6113   3rd Qu.:0.4923
   Max.   :154.0   Max.   :94.00   Max.   :0.6496   Max.   :0.7624
       t_dec
   Min.   :0.08942
   1st Qu.:0.09542
   Median :0.09973
   Mean   :0.10711
   3rd Qu.:0.11393
   Max.   :0.19562
  > summary(read.csv("simplewiki-20.csv"))
      original        encoded          ratio            t_enc
   Min.   :21.00   Min.   :15.00   Min.   :0.4423   Min.   :1.195
   1st Qu.:22.75   1st Qu.:21.00   1st Qu.:0.8047   1st Qu.:1.259
   Median :25.00   Median :24.50   Median :0.9505   Median :1.298
   Mean   :27.15   Mean   :24.93   Mean   :0.9410   Mean   :1.316
   3rd Qu.:29.00   3rd Qu.:28.00   3rd Qu.:1.0804   3rd Qu.:1.346
   Max.   :54.00   Max.   :49.00   Max.   :1.4286   Max.   :1.728
       t_dec
   Min.   :0.1763
   1st Qu.:0.1835
   Median :0.1921
   Mean   :0.2039
   3rd Qu.:0.2189
   Max.   :0.3920
  > summary(read.csv("simplewiki-30.csv"))
      original        encoded          ratio            t_enc
   Min.   :31.00   Min.   :13.00   Min.   :0.2407   Min.   :1.650
   1st Qu.:32.75   1st Qu.:17.75   1st Qu.:0.4718   1st Qu.:1.737
   Median :36.00   Median :24.00   Median :0.6417   Median :1.819
   Mean   :38.85   Mean   :24.36   Mean   :0.6424   Mean   :1.915
   3rd Qu.:40.00   3rd Qu.:29.00   3rd Qu.:0.8000   3rd Qu.:1.900
   Max.   :73.00   Max.   :53.00   Max.   :1.1875   Max.   :4.621
       t_dec
   Min.   :0.2044
   1st Qu.:0.2192
   Median :0.2263
   Mean   :0.2460
   3rd Qu.:0.2499
   Max.   :0.6483
  >