Extend MiniRake::DSL module to prevent inheritance pollution

```include MiniRake::DSL``` on toplevel will pollute ancestor tree of every object.

```extend MiniRake::DSL``` will add those methods only to ```main``` object.

Rake does the same: https://github.com/jimweirich/rake/blob/master/lib/rake/dsl_definition.rb#L153

Added String#sub/sub! and String#gsub/gsub!

Fix typos in compilation guide

string contains null byte check utility function (mrb_str_to_cstr)

Don't overwrite ciidx if it is already set.

add mrbtest verbose mode ('-v' option)

Add documentation for Android toolchain

Fix incorrect index use. nerr was being used for warning buffer check.

Change task rule based on build_dir so main rules don't clobber gem rules

Ignore empty gems

Add semi-colon after the statement.

mrb_any_to_s rewrite

use %p instead of %lx to prevent truncation and for consistency

Generate mrbtest.a file for sparete from driver.o

Add androideabi toolchain rake task (Build mruby with Android NDK)

Modify handling of large fixnum

Fix potential bug about Hash.

Restore arena in `String#split`.

Remove declarations of undefined functions