simple fix for underflow / capture errors and warnings

Fix SEGV when calling Proc object created by Proc.new

Fixed to forget to remove some files on 'make clean'

Ignore files created from building

simple typo in a comment

Remove unnecessary check for size_t < 0

size_t can never be negative since it is an unsigned integer according to standard. Removed to silence warning.
Signed-off-by: Patrick Hogan <pbhogan@gmail.com>

Made text edits to README.md to improve clarity.

fix git commands

#undef DEBUG for compatibility with environments that define DEBUG (like Xcode)

fixed statement alignment

Fixed typos in usage message for mruby tool.

Signed-off-by: Patrick Hogan <pbhogan@gmail.com>