Added some tests for Math module. Some refactoring in math.c; numeric tolerance exported as Math::TOLERANCE and also used for fuzzy tests in mrbtest suite

remove mrb_parse_nstring_ext; use mrb_parser_new() then set p->capture_error = 1; then call mrb_parser_parse(); see tool/mirb/mirb.c

Updated .gitignore for test/

avoid passing NULL to fopen

idx should be less than the cap, if it is equal irep needs to grow.

avoid updating existing libmruby.a to avoid problems on low time resolution file systems; close #136