Feature: Add the possiblity to accept multiple databases from the same backend type

[syphax-bouazzouni]

fix #10 and #30

Changes

• Update the code to accept multiple databases from same backend type (909996e)
• Add source_name property to the results response body (c30e8c1)

http://localhost:8080/api-gateway/search?query=plant&database=earthportal

• Implement filtering to multiple database at once (3b7324c)

http://localhost:8080/api-gateway/search?query=plant&database=tib,zbmed

• Make it possible to use database to select all the services of a type (5a39297)

http://localhost:8080/api-gateway/search?query=plant&database=ontoportal