Improve handling of cmake includes across contract families

[cmickeyb]

One of the nice things about the PDO contracts setup is that we can share contract methods across contract families (e.g. the digital asset family borrows several methods from the token object and token issuer contracts in the exchange family). To handle method definitions the root CMakeLists.txt file explicitly adds a couple of the contract families to the include path. I would like to propose that we remove those explicit paths and replace it with a generic include at the top level of the contract family directory. This would mean changing the CMakeLists.txt in each of the contract families. Specifically, the following kind of change would be necessary:

Before:
INCLUDE(exchange_common)

After:
INCLUDE(exchange-contract/common)

(Note that this presumes that we would also change the name of exchange_common.cmake to common.cmake. The rationale for that is described in a separate issue.)

This change should make it easier for new contract families to share methods without changing the behavior of the project-wide build system. This would simplify independent development of contract families (e.g. including contract families as submodules in the pdo-contract tree).