Implemented tab usage only for indentation (http://llvm.org/PR17363)

Summary:
Changed UseTab to be a enum with three options: Never, Always,
ForIndentation (true/false are still supported when reading .clang-format).
IndentLevel should currently be propagated correctly for all tokens, except for
block comments. Please take a look at the general idea before I start dealing
with block comments.

Reviewers: klimek, djasper

Reviewed By: klimek

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1770