chapter 1, ex 1 through 12
ch1_ex15_a
ch_1_ex_15
1.2.4
renamed to keep in line with naming scheme

--HG--
rename : ch_1_ex_16.scm => ch1_ex16.scm
1.16 finished I think
chapter 1 exercise 17
chapter 1 exercise 18
ch1 ex19
1.2.6 with exercise 1.21 and 1.22
1.23
ch1_ex20
Automated merge with https://bitbucket.org/gertm/sicp/
1.3.1
ch1_29,30,start of 31
ch1_ex31 -> 33
had a bug in ch1_ex33 -- didn't think it through well enough at first
up to 1.37
1.37 done
1.38