LZ4 feature

As Wally wrote to me, "make it optional, make tests and we'll talk in the pull request"
So here it is.
I don't know your coding conventions nor I ever coded C89 in my whole life (only done C++ so far), so I improvised most things.

Also, LZ4 is currently used only on memory values, AOL logs still have uncompressed data, this is because: 

1. It would make more sense to use a slower but better compression algorithm for those, like gzip, since they're read/written not that often.
2. Compressing AOL data would involve some more code around _ol_jar (involving STARTUP checks) which is not hard to write, but I'd rather have this code reviewed before doing even more changes.

Compiles and works on FreeBSD 10.0 x64 and x86 (clang).
Compiles on CentOS (gcc) but the frontend doesn't run.. (liboleg.so missing error) though I get the same error with the main repository, so it's probably not related to my edits.