I replace 'Pre-Conditions' with 'Preconditions' because the former is a ...

add assert check for TravisCI

I correct the mistakes of grammer in README.md. 

I correct the mistakes of grammer in README.md.

Rename --pull_gems to --pull-gems

I fix typos in variable.c.

suppress the warnings from get_valid_iv_sym()

separate out `incremental_gc_step()` from `mrb_incremental_gc()`, and misc minor rename

introduce gc_mark_gray_list(), and cleanup redundant code with it

Fix mrbc error for space included path

The parameter iv_name SHOULD be string or symbol, but there is no
garantee on it.

add comments for Generational Mode in gc.c

Check home is null

If readline is enabled add readline to libraries to link

Move home variable into readline ifdef