[streaming] Several bugfixes and doc updates
[FLINK-1103] [streaming] Added Twitter streaming example and default inputs for examples
[streaming] CoWindow operator rework + stream iteration example added
[FLINK-1032] Rework support for POJO types in the Java API
Added expression keys to distinct and partition operator and addressed some of the comments
Add Pojo support to Scala API
Various fixes
- improved error messages
- the composite vs atomic bug aljoscha found
- Comparator test for Pojo Comparator enabled
- TODO removed
- string-based key expression for group sorting fields definition
- support for specifying "select all" using * and now also _ (for scala fans)
- Exception if user is having multiple fields with the same name in the class
Documentation and fixes
Really add POJO support and nested keys for Scala API

This also adds more integration tests, but not all tests of the Java API
have been ported to Scala yet.
Added wrappers for Hadoop functions

Tests and documentation included. Also tested on cluster.

I hijacked @twalthr's PR #131 to build the documentation of the Hadoop function wrappers on top.

Author: Fabian Hueske <fhueske@apache.org>
Author: Artem Tsikiridis <artem.tsikiridis@cern.ch>
Author: twalthr <info@twalthr.com>

Closes #143 from fhueske/hadoopFunctions and squashes the following commits:

f1b3f21 [Fabian Hueske] [FLINK-1076] Extended Hadoop Compatibility documentation to cover Hadoop functions
f4be4a0 [twalthr] [FLINK-1107] Hadoop Compatibility Layer documented
4a62bdb [Fabian Hueske] [FLINK-1076] Return type determined via type extraction. Added test cases. Minor improvements and clean-ups.
df947d9 [Artem Tsikiridis] [FLINK-1076] Extend Hadoop compatibility. Added wrappers for stand-alone Map, Reduce, and CombinableReduce functions.
Revert "Added wrappers for Hadoop functions"

This reverts commit 74dded1c2cb87c459be3444d1c7387bbc510e154.
[FLINK-1076] Extend Hadoop compatibility. Added wrappers for stand-alone Map, Reduce, and CombinableReduce functions.
[FLINK-1076] Return type determined via type extraction.
Added test cases.
Minor improvements and clean-ups.
[FLINK-1107] Hadoop Compatibility Layer documented

This closes #131
[FLINK-1076] Extended Hadoop Compatibility documentation to cover Hadoop functions
[FLINK-1136] Replaced Flink logo in web submission client
Added Flink logo to JobManager web interface.
Updated Run Example Quickstart screenshots
Removed old gradients and unused image files from submission client.

This closes #145.