Added mongodb as requirement.
Updated bower packages configuration.
references ranking: corrected bug on the map reduce.
tags: corrected bug on the map reduce.
Users: renamed Operator to Other
Users: renamed User to Operator.
Network references: for a peer, if the seed is down the server doesn't crash anymore.
World references: for a peer, if the seed is down the server doesn't crash anymore.
Discovery: for a peer, if the seed is down the server doesn't crash anymore.
Discovery: removed pagination error.
Discovery: the loading icon is not shown anymore when there is an error.
Network references: the loading icon is not shown anymore when there is an error.
World references: the loading icon is not shown anymore when there is an error.
Added mongodb library and updated database instantion code to manage the used database (tingodb or mongodb).
Added singleton object to manage identities. It is able to generate a BSON compatible _id for mongo or standard _id for tingo starting from a generic id.
Updated dataset plugin data retrieve/insertion/update calls to used the new identity system.
Updated peer plugin data retrieve/insertion/update calls to use the new identity system.
Updated reference plugin data retrieve/insertion/update calls to use the new identity system.
Updated repository plugin data retrieve/insertion/update calls to use the new identity system.
Updated setting plugin data retrieve/insertion/update calls to use the new identity system.