Libraries are not linked when running zos push

[spalladino]

It seems that a Solidity library is not being properly linked before the logic contract is pushed to the network.

We could either:
1- Deploy a copy of the library for each logic contract that uses it, which is the fastest way to fix this issue, though limits reusability
2- Have a single copy of the library being shared amongst all contracts, or even exposed as part of an on-chain library, which would require storing it in the contracts object and adding an extra flag to identify it as a linkable library instead of a contract

See #81 for more info

[spalladino]

Related to OpenZeppelin/openzeppelin-labs#91