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.

For now I wrote one test which is basically the unjar_ds one with LZ4 enabled, since the feature doesn't affect Content-Type, keys or expiration, I though it only made sense to test setting/getting values.

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.