Improved API features

• gather taxon id information from taxon name [/get/taxons/{name}] #20 (closed)
• no case sensitivity on searches [/search/taxon;/search/obt] #14 (closed)
• limit the relations in a given document [/search/relations] #13 (closed)
• return several URL for the PubMed record [/get/doc/{source}/{docid}] #16 (closed)
• return several URL for the NCBI taxon [/get/taxon/{taxid};/get/taxons/{name}] #16 (closed)