Unable to install recipes that specify new package-archive for packages installed using ELPA method.

Basically when recipe introduces new element into `package-archives' as value behind key :repo, installation of corresponding package may not occur due to not refreshing archive-contents for new host. This happens if package-archive-contents has been computed during previous iteration of el-get-install when it installed package that didn't have to account for archive. 

In my change, I added explicit check for presence of new address in package-archives, whcih would download it's individual archive-contents and then build summary contents from all archives that are part of current `package-archives'.

The test that I added demonstrates a situation when two packages are being installed in sequence, and the second one relies on ELPA archive, that will only be inside package-archives for the installation of this package.