Add support for groupBy clause

[atulkc]

Library should support GROUP BY clause. User should be able to annotate []string member of struct with groupBy tag. The values in the []string should be the names of the field of the struct that is tagged as selectClause. This will enable developers to take []string as parameter to their method where the users of their library can specify the name of the fields of the struct that is being returned to them on which they desire grouping. Below given is example of how this should work:

type TestSoqlStruct struct {
	SelectClause NonNestedStruct   `soql:"selectClause,tableName=SM_SomeObject__c"`
	WhereClause  TestQueryCriteria `soql:"whereClause"`
	GroupBy   []string                         `soql:"groupByClause"`
}
type TestQueryCriteria struct {
	IncludeNamePattern          []string `soql:"likeOperator,fieldName=Name__c"`
	Roles                                   []string `soql:"inOperator,fieldName=Role__c"`
}
type NonNestedStruct struct {
	Name              string `soql:"selectColumn,fieldName=Name__c"`
	SomeValue     string `soql:"selectColumn,fieldName=SomeValue__c"`
}

To use above structs to create SOQL query

soqlStruct := TestSoqlStruct{
    WhereClause: TestQueryCriteria {
        IncludeNamePattern: []string{"foo", "bar"},
        Roles: []string{"admin", "user"},
    },
   GroupBy: []string{"SomeValue"},
}
soqlQuery, err := Marshal(soqlStruct)
if err != nil {
    fmt.Printf("Error in marshaling: %s\n", err.Error())
}
fmt.Println(soqlQuery)

Above struct will result in following SOQL query:

SELECT Name,SomeValue__c FROM SM_SomeObject__C WHERE (Name__c LIKE '%foo%' OR Name__c LIKE '%bar%') AND Role__c IN ('admin','user') GROUP BY SomeValue___c