Add request_identifier and request_identifier_update methods to Identifier mixin
Add request_identifier and request_identifier_update methods to Identifier mixin

This improves the API for getting and updating identifiers a bit, so you only have to call a few more intuitive method names on the object to get identifiers for that object, by passing a category. Getting the client is handled behind the scenes.

icereval/osf.io,adlius/osf.io,sloria/osf.io,aaxelb/osf.io,adlius/osf.io,felliott/osf.io,mattclark/osf.io,CenterForOpenScience/osf.io,erinspace/osf.io,brianjgeiger/osf.io,HalcyonChimera/osf.io,aaxelb/osf.io,HalcyonChimera/osf.io,brianjgeiger/osf.io,pattisdr/osf.io,brianjgeiger/osf.io,cslzchen/osf.io,baylee-d/osf.io,caseyrollins/osf.io,CenterForOpenScience/osf.io,baylee-d/osf.io,mfraezz/osf.io,aaxelb/osf.io,mattclark/osf.io,felliott/osf.io,erinspace/osf.io,saradbowman/osf.io,caseyrollins/osf.io,cslzchen/osf.io,mfraezz/osf.io,saradbowman/osf.io,baylee-d/osf.io,cslzchen/osf.io,sloria/osf.io,pattisdr/osf.io,brianjgeiger/osf.io,Johnetordoff/osf.io,Johnetordoff/osf.io,icereval/osf.io,mfraezz/osf.io,pattisdr/osf.io,icereval/osf.io,mfraezz/osf.io,cslzchen/osf.io,erinspace/osf.io,caseyrollins/osf.io,felliott/osf.io,HalcyonChimera/osf.io,mattclark/osf.io,Johnetordoff/osf.io,CenterForOpenScience/osf.io,sloria/osf.io,Johnetordoff/osf.io,aaxelb/osf.io,felliott/osf.io,adlius/osf.io,HalcyonChimera/osf.io,adlius/osf.io,CenterForOpenScience/osf.io