Update BUILDING.rst

Split sentences, improve build time estimate.
Merge branch 'building' into DF2014
Merge branch 'cleanup' into DF2014
Merge branch 'gitignore' into DF2014
Merge branch 'readme' into DF2014

Conflicts:
	README.md
Merge branch 'share' into DF2014
Merge branch 'stderr' into DF2014
Merge branch 'travis' into DF2014