ENH: TableExpr.projection now accepts single strings
ENH: TableExpr.projection now accepts single strings

Fixes #413.

korotkyn/ibis,glenioborges/ibis,deepfield/ibis,deepfield/ibis,adamobeng/ibis,laserson/ibis,mahantheshhv/ibis,cpcloud/ibis,korotkyn/ibis,o0neup/ibis,ibis-project/ibis,cpcloud/ibis,Winterflower/ibis,mariusvniekerk/ibis,shaunstanislaus/ibis,cloudera/ibis,korotkyn/ibis,glenioborges/ibis,cpcloud/ibis,aslihandincer/ibis,Winterflower/ibis,dboyliao/ibis,dboyliao/ibis,Winterflower/ibis,ibis-project/ibis,adamobeng/ibis,koverholt/ibis,cpcloud/ibis,adamobeng/ibis,laserson/ibis,Supermem/ibis,ashhher3/ibis,dboyliao/ibis,dalejung/ibis,deepfield/ibis,ibis-project/ibis,cloudera/ibis,shaunstanislaus/ibis,laserson/ibis,dalejung/ibis,mariusvniekerk/ibis,o0neup/ibis,cloudera/ibis,deepfield/ibis,glenioborges/ibis,korotkyn/ibis,ashhher3/ibis,ashhher3/ibis,adamobeng/ibis,o0neup/ibis,aslihandincer/ibis,laserson/ibis,dalejung/ibis,mahantheshhv/ibis,glenioborges/ibis,mariusvniekerk/ibis,wesm/ibis,fivejjs/ibis,Supermem/ibis,fivejjs/ibis,Supermem/ibis,wesm/ibis,ibis-project/ibis,mahantheshhv/ibis,aslihandincer/ibis,fivejjs/ibis,koverholt/ibis,mariusvniekerk/ibis,mahantheshhv/ibis,shaunstanislaus/ibis,aslihandincer/ibis,koverholt/ibis,dboyliao/ibis,ashhher3/ibis,wesm/ibis,koverholt/ibis,Supermem/ibis