Variable specifying library built by project

Adding an external project (with recursive cmake step) adds a variable to let cmake programmers know
that the external project is built by bempp.
variabke is name_BUILT_AS_EXTERNAL_PROJECT
Added messages and MD5 hashes where possible
Adds trilinos as external project (#Issue 60)

Adds an option for mpi. Not sure this is right.
Did not add patch just yet.
Adds root external project to cmake prefix path

This helps ensure newly installed external project are found by cmake.
Needed by Trilinos, for instance.