Merge pull request #338 from dangra/scheme-s3

add s3 scheme as input stream
rebar: Add nowarn_deprecated_type to let Disco compile with Erlang 17.
ddfs master: Refactor ddfs_master to respect the opaqueness of gb_sets().

Basically, instead of using the false|gb_set() for tag_cache we now use
{boolean(), gb_set()} where the first item shows whether the cache is valid or
not.
See: http://erlang.org/pipermail/erlang-questions/2014-April/078647.html
doc: Add a note about Erlang 17.
Merge pull request #490 from pooya/r17

R17
doc: Add a comment about ddfs chunk command.
doc: Add a note about the two different branches.
bump version to 0.5.1.
Merge pull request #491 from pooya/0.5.1

0.5.1
Merge branch 'develop'

Bumping version to 0.5.1
DDFS master node keeps track of blob sizes during garbage collection.
GC peers sends diskinfo to master on startup.
Average disk usage, over- and under-utilized nodes are now computed in ddfs_gc_main:start_gc_phase
NodeStats is updated and log message generated for every diskinfo message.
Optimization by storing the length of NodeStats in a variable.
Combined underused_nodes and overused_nodes into one function.
Added updates of Nodestats on gc_done messages.
Changed find_unstable_nodes to use pattern matching on input.
Added estimate_rr_blobs/1 to estimate effects of rr_blobs.
Renamed variable.