execRunner should exec.

This avoids having a stray bash process that's not useful for anything.
Added -batch mode.
Added .jvmopts support.

If there is a .jvmopts file in the sbt root, these options
will be given to the jvm.  If there is a .sbtopts file, these
will be given to sbt.

Example .jvmopts file for the scala sbt build:

-Xmx4g
-Xms4g
-XX:MaxPermSize=512m
-XX:ReservedCodeCacheSize=256m
Use a per-version -sbt-dir by default.

THIS IS A BREAKING CHANGE IF YOU DEPEND ON PLUGINS IN ~/.sbt.

Tired of being the only guy in town who can't have "always-on" plugins
because I have to build sbt projects with all kinds of different
versions of sbt and they are constantly crashing into one another, I
am selecting a per-version location for property sbt.global.base (aka
-sbt-dir in this runner) of the form ~/.sbt/0.11.2.

I believe it's a far superior default so I'm making it such. If you for
some reason want your plugins found in their former "all sbt versions
all the time" location, you need to add

  -sbt-dir ~/.sbt

to your command line options.

What doesn't work as yet is overriding the version of sbt specified
by a project.  See https://github.com/harrah/xsbt/issues/354 .
Added -q/-quiet option.

Also updated readme.
Respect JAVA_HOME over the PATH configuration (if JAVA_HOME is set).
Updates for 0.12.0.

Somewhat hastily assembled attempt to get working again
with 0.12.0-M1 and the org.scala-sbt group id.
Fixes related to choice of sbt version.

Fixed issues in the I-told-you-it-was-hasty previous commit.
Plus overriding sbt's insistence on using the version in
build.properties the only way we can: updating the file on
disk.  I echoed lots of !!! to make this clear.
sbt: fix relative-path-to-sbt detection
Bug with -scala-version.
New program ivy-classpath.
Better example.
Better error for people without ivy installed.
Fixed group-id for sbt-0.11.3.

Also fixed curl's args so it will fail and not write a file on errors.
Default version 0.11.3, snapshot 0.13.0-SNAPSHOT.
option to configure path for storing sbt launcher jars
Tweaked the launch dir setting a little.

It's called -sbt-launch-dir now.
Adding simple installation instructions.
Fixed typo in help
Added the "universal" sbt launcher.

And cleaned up the echoes, more going to stderr so as not to
compromise output collection.