Update plugins location to ~/.sbt/0.13/plugins/ 

Per https://github.com/sbt/sbt/issues/735
Merge pull request #249 from wsargent/patch-1

Update plugins location to ~/.sbt/0.13/plugins/
Update plugin version in scripted tests to 1.6.0-SNAPSHOT.
Fix filtering of core libraries when using SBT 0.13.0 and Scala != 2.10.2

Before 0.13.0, we *always* used to get JARs from the boot directory; now we
only seem to get these if we use the same Scala version as SBT itself (2.10.2).

This commit relaxes the filtering to handle both naming conventions.
Support any compiler flag that does not need special configuration (#235)

Also, the ones having dedicated tag do not need to be added to general
compilerOptions tag value.
Add notes for 1.5.2
Update sbt and scala versions
Set version 1.5.2
Set development version
Update version in readme