Use the EbuildFetcher class to implement async fetch for the EbuildBuild

Use the EbuildFetcher class to implement async fetch for the EbuildBuild
class. When in background mode, fetch output is logged to emerge-fetch.log.
This solves a problem with calling doebuild(returnpid=1) and then doebuild
calling fetch() synchronously. In that case, doebuild would return an int
if fetch() failed (including when running pkg_nofetch), which would lead
to a TypeError since doebuild(returnpid=1) is supposed to return a list
containing a spawned pid.

svn path=/main/trunk/; revision=11002