[Tools] Minor fixes in release script

- Introduced required RELEASE_BRANCH environment variable
- Some hardcoded binary calls (gpg, sha512sum, md5sum) have been
  replaced by environment variables (GPG, SHASUM, MD5SUM) and added
  "" argument for 'sed -i' calls for compatability with OS X
- Removed unused environment variables
- Directly call ssh/scp instead of going through sshpass
Fix typo in CONTRIBUTORS

Fixes a typo in CONTRIBUTORS, replaces tabs by whitespaces for alignment,
and updates email adresses of Apache committers to @apache.org.
Update CONTRIBUTORS file

Updates CONTRIBUTORS file with contributors from 0.6-incubating release
announcement and sorts by first name.
[FLINK-1097] Fixed multiple slf4j bindings when using Hadoop2

I excluded all slf4j-log4j12 transitive dependencies to get rid off the warning that slf4j finds multiple slf4j bindings in the classpath. Additionally, I added log4j-over-slf4j and excluded all log4j transitive dependencies so that slf4j handles also legacy log4j logging messages from dependencies.

Author: Till Rohrmann <trohrmann@apache.org>

Closes #121 from tillrohrmann/FLINK-1097 and squashes the following commits:

b9f76d5 [Till Rohrmann] Excluded slf4j-log4j12 as transitive dependency in order to not have multiple slf4j bindings in the class path. Added log4j-overslf4j and excluded log4j transitive dependencies to collect log4j outputs.
simple plan for debugging
Merge branch 'runtimeexp' of https://github.com/senorcarbone/incubator-flink into runtimeexp
Merge branch 'master' of https://github.com/apache/incubator-flink into runtimeexp
Remove naive verification of tuple size

This verification will not work.
Think of spaces or escaped ?
I think it's easier for users to parse the SQLException

Author: Moritz Schubotz <github@ckurs.de>

Closes #116 from physikerwelt/patch-1 and squashes the following commits:

69a3d2a [Moritz Schubotz] Remove associated test as well
b51b4f0 [Moritz Schubotz] Remove naive verification of tuple size
Remove space after comma for TupleXX.toString()

It is better to save some bytes when writing the output. Users can always overwrite to `toString()` if they want.

Author: Robert Metzger <rmetzger@apache.org>

Closes #118 from rmetzger/benchFixes and squashes the following commits:

cb8a251 [Robert Metzger] Remove space after comma for TupleXX.toString()
Merge branch 'runtimeexp' of https://github.com/senorcarbone/incubator-flink into runtimeexp
Merge branch 'runtimeexp' of https://github.com/senorcarbone/incubator-flink into runtimeexp
Merge branch 'master' of https://github.com/apache/incubator-flink into runtimeexp
Merge branch 'runtimeexp' of https://github.com/senorcarbone/incubator-flink into runtimeexp