"Contractor, " as used in this chapter, means any individual, firm, partnership, corporation, association or other organization, or any combination thereof, who or which by himself or itself, or by or through others, constructs, alters, repairs, adds to, subtracts from, reconstructs or remodels any building, structure or appurtenance thereto, or who or which undertakes or offers to undertake, or purports to have the capacity to undertake, or submits a bid to do so. The term "contractor" includes a subcontractor and all specialty contractors whose operations as such are the performance of construction work requiring special skills, including, but not limited to, sewer tapping, plumbing, electrical, air conditioning and heating, and whose principal business involves the use of specialized building trades or crafts.