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

"spec.author=" expects a String represents a single author.
"spec.authors=" expects an Array which is a list of multiple authors.

http://guides.rubygems.org/specification-reference/

Before:
Copyright (c) 2013 ["a", "b"]

After:
Copyright (c) 2013 a, b

revise the Two White Parts comments in gc.c

rename advance_phace() to incremental_gc_until(), and cleanup some redundant codes with it.

Performance improvement of methods generated by Module#attr

Fail if dependency isn't found