Added asyncExec call to MRTask2 intended to be used for non-blocking task invocation (either using continuation/callback style or do not wait for the results at all).

The functionality is basically the same as of dfork call, except the invoked task will run at the same priority level as the calling thread or min_priority if called outside of FJ framework (whereas dfork automatically increases the priority level to avoid deadlock).

Adapted GLM2 (and Parse2) to use this call instead of dfork so that every GLM2 iteration runs at the same priority level (GLM2 launches iterations from FJ thread but uses continuation style callback and not blocking, so there is no risk of deadlock).
Merge remote-tracking branch 'origin/master'
Sparse chunk update.

Added CXIChunk for sparse integer: contains 2,4 or 8 byte values run-length encoded with skip (row-id) having 2 or 4 bytes.
Added CX0Chunk - subclass of CXIChunk specialized for binary sparse vectors with no NAs (contains only row-ids, no values).
Added CXDChunk - subclass of CXIChunk for sparse floats (values always encoded as Doubles)
Removed CX2Chunk - replaced by CXIChunk.

Updated NewChunk to store numbers as sparse by default (added _id array containing row-ids of non-zero elements and not storing zeros).
Update of the  sparse chunks.
Merge branch 'master' of github.com:0xdata/h2o

Conflicts:
	src/main/java/water/MRTask2.java
	src/main/java/water/api/Inspect2.java
Added SparseTest for testing of sparse chunks- compression and get/set and get-with-skip-count(iteration only over non-zeros)
Merge branch 'master' of github.com:0xdata/h2o
Sparse chunks update.
Merge branch 'master' of github.com:0xdata/h2o