Support GNU style rule to put a space before opening parenthesis.

Summary:
The rule from the GNU style states:
"We find it easier to read a program when it has spaces before the open-parentheses and after the commas."

http://www.gnu.org/prep/standards/standards.html#index-spaces-before-open_002dparen

This patch makes clang-format adds an option to put spaces before almost all open parentheses, except the cases, where different behavior is dictated by the style rules or language syntax:
  * preprocessor:
    ** function-like macro definitions can't have a space between the macro name and the parenthesis;
    ** `#if defined(...)` can have a space, but it seems, that it's more frequently used without a space in GCC, for example;
  * never add spaces after unary operators;
  * adding spaces between two opening parentheses is controlled with the `SpacesInParentheses` option;
  * never add spaces between `[` and `(` (there's no option yet).

Reviewers: djasper

Reviewed By: djasper

CC: cfe-commits, klimek

Differential Revision: http://llvm-reviews.chandlerc.com/D2326