Highlighting shouldn't fail when the field to highlight is absent.

PlainHighlighter fails with a NPE when the field to highlight is marked as
stored in the mapping but doesn't exist in a hit. This patch makes
FieldsVisitor.fields less error-prone by returning an empty list instead
of null when no matching stored field was found.

Closes #3109
Store _version as a numeric doc values field.

Doc values can be expected to be more compact than payloads and should provide
better flexibility since doc values formats can be picked on a per-field basis.
This patch:
 - makes _version stored as a numeric doc values field,
 - manages backwards compatibility: if a version is not found in doc values,
   then it will look into payloads,
 - uses background merges to upgrade old segments and move _version from
   payloads to doc values.

Closes #3103
add 0.90.2 version
Make it easier to get started with Eclipse.

This patch makes mvn eclipse:eclipse generate additional eclipse configuration
files so that Eclipse:
 - uses Java 1.6 compliance level,
 - truncates lines after 140 chars,
 - uses 4 spaces for indentation,
 - automatically adds a license header when creating a new class file,
 - organizes imports the same way as Intellij Idea (which makes sense I guess
   since most of the code bas has been written with Intellij, this will prevent
   from having large diffs due to the fact that the order of imports has
   changed).
Add more information and options in PluginManager
New option -l, --list displays list of existing plugins
New option -h, --help displays help
Deprecate options:
   -install is now -i, --install
   -remove is now -r, --remove
   -url is now -u, --url
Catch ArraysOutOfBoundException when no arg given to install, remove or url option
Add description on plugin name structure:
- elasticsearch/plugin/version for official elasticsearch plugins (download from download.elasticsearch.org)
- groupId/artifactId/version   for community plugins (download from maven central or oss sonatype)
- username/repository          for site plugins (download from github master)
Closes #3112.
Allow negative slops in SpanNearQueryParser

This is mainly due to the fact that SpanNearQuery allows some neat
tricks with negative slops to run zero-sloped near queries across
2 or more SpanTermQueries.

Closes #3079
Improve integration testing by reusing an abstracted cluster across tests

The new AbstractSharedClusterTest abstracts integration testing further to
reduce the overhead of writing tests that don't rely on explict control over
the cluster. For instance tests that run query, facets or that test highlighting
don't need to explictly start and stop nodes. Testing features like the ones
just mentioned are based on the assumption that the underlying cluster can
be arbitray. Based on this assumption this base class allows to:

 * randomize cluster and index settings if not explictly specified
 * transparently test transport & node clients
 * test features like search or highlighting on different cluster sizes
 * allow reuse of node insteance across tests
 * provide utility methods that act as upper or lower bounds that a test must pass with
   ie. if a test requries at least 3 nodes then it should also pass with 4 nodes
 * given a cluster has unmodified cluster settings (persistent and transient) the cluster
   should not differ to a fresh started cluster when reused across nodes.
 * within a test the client implementation and the clients associated node can be changed
   at any time and should return a valid result.

This patch also prepares some redundant tests like 'RelocationTests.java' for randomized
testing. Test like this are very long-running on some machines and run the same test with
different parameters like 'number of writers' or 'number of relocations' which can easily
be chosen with a random number and run only ones during development but multiple times
during CI builds.

All the improvements in this change reduce the test time by ~30%
Merge branch 'master' of https://github.com/elasticsearch/elasticsearch