Fixed small running on YARN docs typo

The backslash is needed for multiline command

Author: Vlad <frolvlad@gmail.com>

Closes #1158 from frol/patch-1 and squashes the following commits:

e258044 [Vlad] Fixed small running on YARN docs typo
Fix mvn detection

When mvn is not detected (not in executor's path), 'set -e' causes the
detection to terminate the script before the helpful error message can
be displayed.

Author: Matthew Farrellee <matt@redhat.com>

Closes #1181 from mattf/master-0 and squashes the following commits:

506549f [Matthew Farrellee] Fix mvn detection
[SPARK-1669][SQL]  Made cacheTable idempotent

JIRA issue: [SPARK-1669](https://issues.apache.org/jira/browse/SPARK-1669)

Caching the same table multiple times should end up with only 1 in-memory columnar representation of this table.

Before:

```
scala> loadTestTable("src")
...
scala> cacheTable("src")
...
scala> cacheTable("src")
...
scala> table("src")
...
== Query Plan ==
InMemoryColumnarTableScan [key#2,value#3], (InMemoryRelation [key#2,value#3], false, (InMemoryColumnarTableScan [key#2,value#3], (InMemoryRelation [key#2,value#3], false, (HiveTableScan [key#2,value#3], (MetastoreRelation default, src, None), None))))
```

After:

```
scala> loadTestTable("src")
...
scala> cacheTable("src")
...
scala> cacheTable("src")
...
scala> table("src")
...
== Query Plan ==
InMemoryColumnarTableScan [key#2,value#3], (InMemoryRelation [key#2,value#3], false, (HiveTableScan [key#2,value#3], (MetastoreRelation default, src, None), None))
```

Author: Cheng Lian <lian.cs.zju@gmail.com>

Closes #1183 from liancheng/spark-1669 and squashes the following commits:

68f8a20 [Cheng Lian] Removed an unused import
51bae90 [Cheng Lian] Made cacheTable idempotent
[SPARK-2118] spark class should complain if tools jar is missing.

Author: Prashant Sharma <prashant.s@imaginea.com>

Closes #1068 from ScrapCodes/SPARK-2118/tools-jar-check and squashes the following commits:

29e768b [Prashant Sharma] Code Review
5cb6f7d [Prashant Sharma] [SPARK-2118] spark class should complaing if tools jar is missing.
[SPARK-1768] History server enhancements.

Two improvements to the history server:

- Separate the HTTP handling from history fetching, so that it's easy to add
  new backends later (thinking about SPARK-1537 in the long run)

- Avoid loading all UIs in memory. Do lazy loading instead, keeping a few in
  memory for faster access. This allows the app limit to go away, since holding
  just the listing in memory shouldn't be too expensive unless the user has millions
  of completed apps in the history (at which point I'd expect other issues to arise
  aside from history server memory usage, such as FileSystem.listStatus()
  starting to become ridiculously expensive).

I also fixed a few minor things along the way which aren't really worth mentioning.
I also removed the app's log path from the UI since that information may not even
exist depending on which backend is used (even though there is only one now).

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #718 from vanzin/hist-server and squashes the following commits:

53620c9 [Marcelo Vanzin] Add mima exclude, fix scaladoc wording.
c21f8d8 [Marcelo Vanzin] Feedback: formatting, docs.
dd8cc4b [Marcelo Vanzin] Standardize on using spark.history.* configuration.
4da3a52 [Marcelo Vanzin] Remove UI from ApplicationHistoryInfo.
2a7f68d [Marcelo Vanzin] Address review feedback.
4e72c77 [Marcelo Vanzin] Remove comment about ordering.
249bcea [Marcelo Vanzin] Remove offset / count from provider interface.
ca5d320 [Marcelo Vanzin] Remove code that deals with unfinished apps.
6e2432f [Marcelo Vanzin] Second round of feedback.
b2c570a [Marcelo Vanzin] Make class package-private.
4406f61 [Marcelo Vanzin] Cosmetic change to listing header.
e852149 [Marcelo Vanzin] Initialize new app array to expected size.
e8026f4 [Marcelo Vanzin] Review feedback.
49d2fd3 [Marcelo Vanzin] Fix a comment.
91e96ca [Marcelo Vanzin] Fix scalastyle issues.
6fbe0d8 [Marcelo Vanzin] Better handle failures when loading app info.
eee2f5a [Marcelo Vanzin] Ensure server.stop() is called when shutting down.
bda2fa1 [Marcelo Vanzin] Rudimentary paging support for the history UI.
b284478 [Marcelo Vanzin] Separate history server from history backend.
Cleanup on Connection, ConnectionManagerId, ConnectionManager classes part 2

Cleanup on Connection, ConnectionManagerId, and ConnectionManager classes part 2 while I was working at the code there to help IDE:
1. Remove unused imports
2. Remove parentheses in method calls that do not have side affect.
3. Add parentheses in method calls that do have side effect or not simple get to object properties.
4. Change if-else check (via isInstanceOf) for Connection class type with Scala expression for consistency and cleanliness.
5. Remove semicolon
6. Remove extra spaces.
7. Remove redundant return for consistency

Author: Henry Saputra <henry.saputra@gmail.com>

Closes #1157 from hsaputra/cleanup_connection_classes_part2 and squashes the following commits:

4be6906 [Henry Saputra] Fix Spark Scala style for line over 100 chars.
85b24f7 [Henry Saputra] Cleanup on Connection and ConnectionManager classes part 2 while I was working at the code there to help IDE: 1. Remove unused imports 2. Remove parentheses in method calls that do not have side affect. 3. Add parentheses in method calls that do have side effect. 4. Change if-else check (via isInstanceOf) for Connection class type with Scala expression for consitency and cleanliness. 5. Remove semicolon 6. Remove extra spaces.
[SPARK-2227] Support dfs command in SQL.

Note that nothing gets printed to the console because we don't properly maintain session state right now.

I will have a followup PR that fixes it.

Author: Reynold Xin <rxin@apache.org>

Closes #1167 from rxin/commands and squashes the following commits:

56f04f8 [Reynold Xin] [SPARK-2227] Support dfs command in SQL.
Merge branch 'master' of https://github.com/apache/spark into SPARK-1291_bug
bugfix