Add reference to Blazejowski et al.(2004) in Sec 2

[jbudynk]

\bibitem[B{\l}a{.z}ejowski et al.(2004)]{2004ApJ...600L..27B} B{\l}a{.z}ejowski, M., Siemiginowska, A., Sikora, M., Moderski, R., & Bechtold, J.\ 2004, \apjl, 600, L27

[jbudynk]

Also add more references to Sherpa:

\bibitem[Freeman et al.(2001)]{2001SPIE.4477...76F} Freeman, P., Doe, S.,
& Siemiginowska, A.\ 2001, \procspie, 4477, 76

\bibitem[Refsdal et al.(2009)]{2009pysc.conf...51R} Refsdal, B.~L., Doe,
S.~M., Nguyesn, D.~T., et al.\ 2009, Proceedings of the 8th Python in
Science Conference, Pasadena, CA, 2009, edited by G.~Varoquaux, S.~van der
Walt and J.~Millman, 51