Support Ivy 2.3.0-final.

This entailed modifying ResolutionCache and the CustomPomParser
to reflect changes to the ResolutionCacheManager interface and
DefaultExtendsDescriptor class between Ivy 2.3.0-rc1 and
2.3.0-rc2. Specifically,

1. ResolutionCacheManager now includes two additional methods
that needed implementations in ResolutionCache:
getResolvedModuleDescriptor(mrid: ModuleRevisionId) and
saveResolvedModuleDescriptor(md: ModuleDescriptor). I adapted
the implementations for these (which are expressed primarily in
terms of other interface methods) from Ivy 2.3.0's
DefaultResolutionCacheManager.

2. Instead of taking a ModuleRevisionIdentifier and a resolved
ModuleRevisionIdentifier as its first two arguments, the
DefaultExtendsDescriptor constructor now takes a
ModuleDescriptor. This was a trivial change.

Note that ResolutionCache.getResolvedModuleDescriptor does not
appear to be used by Ivy as sbt uses Ivy and there is thus no
test coverage for its implementation. Also note that the
DefaultResolutionCacheManager object created in
Update.configureResolutionCache now requires a reference to an
IvySettings object; DRCM expects this to be non-null.
Update documentation references to Ivy 2.3.0