Updated to new Python core

Updated python core version and source location
Base is building for 2.7.6, testing needed
Updated installer version strings for all packages
More package updates

More package updates + splashscreen update for 2.7
Fixed pandas and matplotlib

Pandas and matplotlib needed additional dependencies, added logic in the
package selector for unselecting dependent packages
Added xlrd, xlwt and xlutils + some import tests
Added openpyxl support
Updated lxml to the latest version
Merge remote-tracking branch 'origin/2.7.6.1'