added 2 books (angrist & pischke, imbens & rubin)

Added: Angrist and Pischke's "Mastering 'Metrics" and Imbens and Rubin's "Causal Inference in Statistics, Social, and Biomedical Sciences". Fixed a typo (Pishke = Pischke).
removed duplicated entry (Freedman, 1991)
added synthetic control method
Merge branch 'master' of https://github.com/danilofreire/causalinference into danilofreire-master

Conflicts:
	books.md
	general.md
Merge branch 'danilofreire-master'