As the nation’s job market continues to improve, albeit more slowly than many would like, a fresh crop of employment Web sites are coming online to connect niche groups of job seekers with vacant positions. RemarkableHire, a Northern Virginia upstart with software still under development, is among the latest outfits. Co-founded by Scott Rothrock and Jamey Jeff, the company aims to help employers scour Web sites and forums where techies congregate to find skilled engineers and developers.